Investment Securities, AFS and HTM Securities Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investment securities available for sale:
Amortized Cost	$ 4,584,763	$ 4,100,417
Gross Unrealized Gains	9,186	10,996
Gross Unrealized Losses	(108,076)	(67,967)
Fair Value	4,485,875	4,043,446
Investment securities held to maturity:
Amortized Cost	2,443,203	2,282,853
Gross Unrealized Gains	15,555	26,543
Gross Unrealized Losses	(51,561)	(25,822)
Fair Value	2,407,197	2,283,574
US Treasury Securities
Investment securities available for sale:
Amortized Cost	1,002	1,003
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(7)	(7)
Fair Value	995	996
Obligations of state and political subdivisions (municipal securities)
Investment securities held to maturity:
Amortized Cost	1,424,925	1,281,320
Gross Unrealized Gains	4,110	13,899
Gross Unrealized Losses	(16,579)	(3,177)
Fair Value	1,412,456	1,292,042
Private Label
Investment securities available for sale:
Amortized Cost	1,051	1,067
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(7)	(8)
Fair Value	1,044	1,059
Investment securities held to maturity:
Amortized Cost	0
Fair Value	0
GNMA Commercial Mortgage-Related Securities
Investment securities available for sale:
Amortized Cost	1,497,573	1,547,173
Gross Unrealized Gains	0	5
Gross Unrealized Losses	(49,204)	(33,901)
Fair Value	1,448,369	1,513,277
Investment securities held to maturity:
Amortized Cost	531,934
Gross Unrealized Gains	9,008
Gross Unrealized Losses	(21,694)
Fair Value	519,248
Asset backed Securities
Investment securities available for sale:
Amortized Cost	288,519
Gross Unrealized Gains	1,835
Gross Unrealized Losses	(54)
Fair Value	290,300
Investment securities held to maturity:
Amortized Cost	0
Fair Value	0
Other Debt And Other Equity Securities
Investment securities available for sale:
Amortized Cost	3,200	3,200
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(12)	(12)
Fair Value	3,188	3,188
Equity Securities [Member]
Investment securities available for sale:
Amortized Cost	1,599	1,519
Gross Unrealized Gains	0	127
Gross Unrealized Losses	0	(14)
Fair Value	1,599	1,632
Investment securities held to maturity:
Amortized Cost	0
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
FNMA and FHLMC \| Residential Related Securities
Investment securities available for sale:
Amortized Cost	427,480	457,680
Gross Unrealized Gains	7,331	9,722
Gross Unrealized Losses	(4,738)	(2,634)
Fair Value	430,073	464,768
Investment securities held to maturity:
Amortized Cost	76,069	40,995
Gross Unrealized Gains	227	398
Gross Unrealized Losses	(1,665)	(489)
Fair Value	74,631	40,904
GNMA \| Residential Related Securities
Investment securities available for sale:
Amortized Cost	2,364,339	1,944,453
Gross Unrealized Gains	21	275
Gross Unrealized Losses	(54,054)	(31,378)
Fair Value	2,310,306	1,913,350
Investment securities held to maturity:
Amortized Cost	410,275	414,440
Gross Unrealized Gains	2,210	2,700
Gross Unrealized Losses	(11,624)	(6,400)
Fair Value	$ 400,861	410,740
GNMA \| GNMA Commercial Mortgage-Related Securities
Investment securities held to maturity:
Amortized Cost		546,098
Gross Unrealized Gains		9,546
Gross Unrealized Losses		(15,756)
Fair Value		$ 539,888

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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