Quarterly report pursuant to Section 13 or 15(d)

AVAILABLE-FOR-SALE AND OTHER SECURITIES - Contractual Maturities (Details)

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AVAILABLE-FOR-SALE AND OTHER SECURITIES - Contractual Maturities (Details) - USD ($)
$ in Thousands
Mar. 31, 2016
Dec. 31, 2015
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost $ 9,238,387 $ 8,770,923
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Available-for-sale and other securities 9,319,381 8,775,441
U.S. Treasury    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost, Under 1 year 798 0
Amortized Cost, 1-5 years 5,463 5,457
Amortized Cost, 6-10 years 0 0
Amortized Cost, Over 10 years 0 0
Amortized Cost 6,261 5,457
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 798 0
Fair Value, 1-5 years 5,509 5,472
Fair Value, 6-10 years 0 0
Fair Value, Over 10 years 0 0
Available-for-sale and other securities 6,307 5,472
Federal agencies: Mortgage-backed securities    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost, Under 1 year 51,065 51,146
Amortized Cost, 1-5 years 104,732 111,655
Amortized Cost, 6-10 years 241,927 254,397
Amortized Cost, Over 10 years 4,371,145 4,088,120
Amortized Cost 4,768,869 4,505,318
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 50,983 51,050
Fair Value, 1-5 years 106,924 113,393
Fair Value, 6-10 years 247,936 257,765
Fair Value, Over 10 years 4,436,149 4,099,480
Available-for-sale and other securities 4,841,992 4,521,688
Federal agencies, Other agencies    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost, Under 1 year 2,202 801
Amortized Cost, 1-5 years 8,146 9,101
Amortized Cost, 6-10 years 101,107 105,174
Amortized Cost, Over 10 years 0 0
Amortized Cost 111,455 115,076
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 2,242 805
Fair Value, 1-5 years 8,549 9,395
Fair Value, 6-10 years 103,705 105,713
Fair Value, Over 10 years 0 0
Available-for-sale and other securities 114,496 115,913
Total U.S. Treasury, Federal agency securities    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost 4,886,585 4,625,851
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Available-for-sale and other securities 4,962,795 4,643,073
Municipal securities    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost, Under 1 year 285,763 281,644
Amortized Cost, 1-5 years 587,570 587,664
Amortized Cost, 6-10 years 1,164,206 1,053,502
Amortized Cost, Over 10 years 557,927 509,133
Amortized Cost 2,595,466 2,431,943
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 275,832 280,823
Fair Value, 1-5 years 592,057 587,345
Fair Value, 6-10 years 1,171,111 1,048,550
Fair Value, Over 10 years 594,756 539,678
Available-for-sale and other securities 2,633,756 2,456,396
Asset-backed securities    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost, Under 1 year 0 0
Amortized Cost, 1-5 years 150,053 110,115
Amortized Cost, 6-10 years 88,267 128,342
Amortized Cost, Over 10 years 641,296 662,602
Amortized Cost 879,616 901,059
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 0 0
Fair Value, 1-5 years 149,844 109,300
Fair Value, 6-10 years 89,019 128,208
Fair Value, Over 10 years 598,511 623,905
Available-for-sale and other securities 837,374 861,413
Corporate debt    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost, Under 1 year 29,307 300
Amortized Cost, 1-5 years 346,591 356,513
Amortized Cost, 6-10 years 152,382 107,394
Amortized Cost, Over 10 years 0 0
Amortized Cost 528,280 464,207
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 29,919 302
Fair Value, 1-5 years 353,571 360,653
Fair Value, 6-10 years 153,171 105,522
Fair Value, Over 10 years 0 0
Available-for-sale and other securities 536,661 466,477
Other securities    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost, Under 1 year 0 0
Amortized Cost, 1-5 years 3,950 3,950
Amortized Cost, 6-10 years 0 0
Amortized Cost, Over 10 years 0 0
Amortized Cost 348,440 347,863
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 0 0
Fair Value, 1-5 years 3,931 3,898
Fair Value, 6-10 years 0 0
Fair Value, Over 10 years 0 0
Available-for-sale and other securities 348,795 348,082
Non-marketable equity securities    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost 333,460 332,786
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Available-for-sale and other securities 333,460 332,786
Mutual funds    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost 10,506 10,604
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Available-for-sale and other securities 10,506 10,604
Marketable equity securities    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost 524 523
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Available-for-sale and other securities $ 898 $ 794