Investment Securities, AFS and HTM Securities Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investment securities available for sale:
Amortized Cost	$ 4,377,443	$ 4,100,417
Gross Unrealized Gains	7,275	10,996
Gross Unrealized Losses	(123,068)	(67,967)
Fair Value	4,261,651	4,043,446
Held to Maturity Amortized Cost	2,602,247	2,282,853
Held-to-maturity, Gross Unrealized Gains	15,553	26,543
Held-to-maturity, Gross Unrealized Loss	(56,856)	(25,822)
Held-to-maturity, Fair Value	2,560,943	2,283,574
US Treasury Securities
Investment securities available for sale:
Amortized Cost	1,001	1,003
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(6)	(7)
Fair Value	996	996
Obligations of state and political subdivisions (municipal securities)
Investment securities available for sale:
Held to Maturity Amortized Cost	1,593,147	1,281,320
Held-to-maturity, Gross Unrealized Gains	4,851	13,899
Held-to-maturity, Gross Unrealized Loss	(17,698)	(3,177)
Held-to-maturity, Fair Value	1,580,300	1,292,042
Residential Related Securities \| FNMA and FHLMC
Investment securities available for sale:
Amortized Cost	396,368	457,680
Gross Unrealized Gains	5,573	9,722
Gross Unrealized Losses	(5,232)	(2,634)
Fair Value	396,709	464,768
Held to Maturity Amortized Cost	97,822	40,995
Held-to-maturity, Gross Unrealized Gains	191	398
Held-to-maturity, Gross Unrealized Loss	(2,306)	(489)
Held-to-maturity, Fair Value	95,707	40,904
Residential Related Securities \| GNMA
Investment securities available for sale:
Amortized Cost	2,275,481	1,944,453
Gross Unrealized Gains	23	275
Gross Unrealized Losses	(65,315)	(31,378)
Fair Value	2,210,189	1,913,350
Held to Maturity Amortized Cost	388,174	414,440
Held-to-maturity, Gross Unrealized Gains	1,969	2,700
Held-to-maturity, Gross Unrealized Loss	(13,020)	(6,400)
Held-to-maturity, Fair Value	377,123	410,740
Private Label
Investment securities available for sale:
Amortized Cost	1,037	1,067
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(5)	(8)
Fair Value	1,031	1,059
Held to Maturity Amortized Cost	0
Held-to-maturity, Fair Value	0
GNMA Commercial Mortgage-Related Securities
Investment securities available for sale:
Amortized Cost	1,400,680	1,547,173
Gross Unrealized Gains	0	5
Gross Unrealized Losses	(52,399)	(33,901)
Fair Value	1,348,282	1,513,277
Held to Maturity Amortized Cost	523,103
Held-to-maturity, Gross Unrealized Gains	8,542
Held-to-maturity, Gross Unrealized Loss	(23,832)
Held-to-maturity, Fair Value	507,813
GNMA Commercial Mortgage-Related Securities \| GNMA
Investment securities available for sale:
Held to Maturity Amortized Cost		546,098
Held-to-maturity, Gross Unrealized Gains		9,546
Held-to-maturity, Gross Unrealized Loss		(15,756)
Held-to-maturity, Fair Value		539,888
Asset backed Securities
Investment securities available for sale:
Amortized Cost	298,263
Gross Unrealized Gains	1,679
Gross Unrealized Losses	(111)
Fair Value	299,831
Held to Maturity Amortized Cost	0
Held-to-maturity, Fair Value	0
Other Debt And Other Equity Securities
Investment securities available for sale:
Amortized Cost	3,000	3,200
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(12)
Fair Value	3,000	3,188
Equity Securities [Member]
Investment securities available for sale:
Amortized Cost	1,613	1,519
Gross Unrealized Gains	0	127
Gross Unrealized Losses	0	(14)
Fair Value	1,613	1,632
Held to Maturity Amortized Cost	0
Held-to-maturity, Fair Value	0
Federal Family Education Loan Program (FFELP) Guaranteed Loans [Member]
Investment securities available for sale:
Amortized Cost		144,322
Gross Unrealized Gains		867
Gross Unrealized Losses		(13)
Fair Value		$ 145,176
Investment Securities Class [Domain]
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Securities Received as Collateral	$ 3,100,000

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