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
Jun. 30, 2016
Dec. 31, 2015
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost $ 9,522,960 $ 8,770,923
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Available-for-sale and other securities 9,653,038 8,775,441
U.S. Treasury    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost, Under 1 year 1,798 0
Amortized Cost, 1-5 years 5,468 5,457
Amortized Cost, 6-10 years 0 0
Amortized Cost, Over 10 years 0 0
Amortized Cost 7,266 5,457
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 1,799 0
Fair Value, 1-5 years 5,521 5,472
Fair Value, 6-10 years 0 0
Fair Value, Over 10 years 0 0
Available-for-sale and other securities 7,320 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 51,000 51,146
Amortized Cost, 1-5 years 96,565 111,655
Amortized Cost, 6-10 years 239,445 254,397
Amortized Cost, Over 10 years 4,734,778 4,088,120
Amortized Cost 5,121,788 4,505,318
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 50,982 51,050
Fair Value, 1-5 years 98,664 113,393
Fair Value, 6-10 years 246,718 257,765
Fair Value, Over 10 years 4,821,428 4,099,480
Available-for-sale and other securities 5,217,792 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 1,650 801
Amortized Cost, 1-5 years 7,494 9,101
Amortized Cost, 6-10 years 73,899 105,174
Amortized Cost, Over 10 years 0 0
Amortized Cost 83,043 115,076
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 1,688 805
Fair Value, 1-5 years 7,883 9,395
Fair Value, 6-10 years 76,422 105,713
Fair Value, Over 10 years 0 0
Available-for-sale and other securities 85,993 115,913
Total U.S. Treasury, Federal agency securities    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost 5,212,097 4,625,851
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Available-for-sale and other securities 5,311,105 4,643,073
Municipal securities    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost, Under 1 year 316,563 281,644
Amortized Cost, 1-5 years 699,471 587,664
Amortized Cost, 6-10 years 1,029,450 1,053,502
Amortized Cost, Over 10 years 488,761 509,133
Amortized Cost 2,534,245 2,431,943
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 306,297 280,823
Fair Value, 1-5 years 707,937 587,345
Fair Value, 6-10 years 1,049,301 1,048,550
Fair Value, Over 10 years 518,092 539,678
Available-for-sale and other securities 2,581,627 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 174,999 110,115
Amortized Cost, 6-10 years 88,174 128,342
Amortized Cost, Over 10 years 656,860 662,602
Amortized Cost 920,033 901,059
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 0 0
Fair Value, 1-5 years 176,453 109,300
Fair Value, 6-10 years 90,210 128,208
Fair Value, Over 10 years 623,909 623,905
Available-for-sale and other securities 890,572 861,413
Corporate debt    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost, Under 1 year 94,200 300
Amortized Cost, 1-5 years 346,755 356,513
Amortized Cost, 6-10 years 66,337 107,394
Amortized Cost, Over 10 years 0 0
Amortized Cost 507,292 464,207
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 95,772 302
Fair Value, 1-5 years 355,944 360,653
Fair Value, 6-10 years 68,288 105,522
Fair Value, Over 10 years 0 0
Available-for-sale and other securities 520,004 466,477
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 349,293 347,863
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 0 0
Fair Value, 1-5 years 3,947 3,898
Fair Value, 6-10 years 0 0
Fair Value, Over 10 years 0 0
Available-for-sale and other securities 349,730 348,082
Non-marketable equity securities    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost 333,751 332,786
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Available-for-sale and other securities 333,751 332,786
Mutual funds    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost 11,069 10,604
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Available-for-sale and other securities 11,069 10,604
Marketable equity securities    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost 523 523
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Available-for-sale and other securities $ 963 $ 794