Investment Securities, AFS and HTM Securities Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investment securities available for sale:
Amortized Cost	$ 4,191,366	$ 4,100,417
Gross Unrealized Gains	4,424	10,996
Gross Unrealized Losses	(141,593)	(67,967)
Fair Value	4,054,197	4,043,446
Debt Securities, Held-to-maturity
Held to Maturity, Amortized Cost	2,661,755	2,282,853
Held-to-maturity, Gross Unrealized Gains	11,223	26,543
Held-to-maturity, Gross Unrealized Loss	(77,961)	(25,822)
Held-to-maturity, Fair Value	2,595,017	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	(4)	(7)
Fair Value	997	996
Obligations of state and political subdivisions (municipal securities)
Debt Securities, Held-to-maturity
Held to Maturity, Amortized Cost	1,682,162	1,281,320
Held-to-maturity, Gross Unrealized Gains	1,216	13,899
Held-to-maturity, Gross Unrealized Loss	(33,332)	(3,177)
Held-to-maturity, Fair Value	1,650,046	1,292,042
Residential Related Securities \| FNMA/FHLMC
Investment securities available for sale:
Amortized Cost	320,148	457,680
Gross Unrealized Gains	2,658	9,722
Gross Unrealized Losses	(6,132)	(2,634)
Fair Value	316,674	464,768
Debt Securities, Held-to-maturity
Held to Maturity, Amortized Cost	95,603	40,995
Held-to-maturity, Gross Unrealized Gains	162	398
Held-to-maturity, Gross Unrealized Loss	(3,266)	(489)
Held-to-maturity, Fair Value	92,499	40,904
Residential Related Securities \| GNMA
Investment securities available for sale:
Amortized Cost	2,259,538	1,944,453
Gross Unrealized Gains	0	275
Gross Unrealized Losses	(79,528)	(31,378)
Fair Value	2,180,010	1,913,350
Debt Securities, Held-to-maturity
Held to Maturity, Amortized Cost	367,536	414,440
Held-to-maturity, Gross Unrealized Gains	1,753	2,700
Held-to-maturity, Gross Unrealized Loss	(15,337)	(6,400)
Held-to-maturity, Fair Value	353,952	410,740
Private-label
Investment securities available for sale:
Amortized Cost	1,022	1,067
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(4)	(8)
Fair Value	1,018	1,059
Debt Securities, Held-to-maturity
Held to Maturity, Amortized Cost	0
Held-to-maturity, Fair Value	0
GNMA Commercial Mortgage-Related Securities
Investment securities available for sale:
Amortized Cost	1,306,802	1,547,173
Gross Unrealized Gains	0	5
Gross Unrealized Losses	(55,824)	(33,901)
Fair Value	1,250,978	1,513,277
Debt Securities, Held-to-maturity
Held to Maturity, Amortized Cost	516,454	546,098
Held-to-maturity, Gross Unrealized Gains	8,092	9,546
Held-to-maturity, Gross Unrealized Loss	(26,026)	(15,756)
Held-to-maturity, Fair Value	498,520	539,888
FFELP asset backed securities
Investment securities available for sale:
Amortized Cost	298,282	144,322
Gross Unrealized Gains	1,766	867
Gross Unrealized Losses	(101)	(13)
Fair Value	299,947	145,176
Debt Securities, Held-to-maturity
Held to Maturity, Amortized Cost	0
Held-to-maturity, Fair Value	0
Other Debt 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
Other Equity Securities
Investment securities available for sale:
Amortized Cost	1,573	1,519
Gross Unrealized Gains	0	127
Gross Unrealized Losses	0	(14)
Fair Value	1,573	1,632
Debt Securities, Held-to-maturity
Held to Maturity, Amortized Cost	0
Held-to-maturity, Fair Value	0
Investment Securities Class
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Securities Received as Collateral	$ 3,700,000	$ 3,100,000

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