Investment Securities, AFS and HTM Securities Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment securities available for sale
Amortized Cost	$ 4,100,417	$ 4,724,895
Gross Unrealized Gains	10,996	19,132
Gross Unrealized Losses	(67,967)	(63,801)
Fair Value	4,043,446	4,680,226
Investment securities held to maturity
Amortized Cost	2,282,853	1,273,536
Gross Unrealized Gains	26,543	4,523
Gross Unrealized Losses	(25,822)	(13,385)
Fair Value	2,283,574	1,264,674
U.S. Treasury securities
Investment securities available for sale
Amortized Cost	1,003	1,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(7)	0
Fair Value	996	1,000
Obligations of state and political subdivisions (municipal securities)
Investment securities held to maturity
Amortized Cost	1,281,320	1,145,843
Gross Unrealized Gains	13,899	3,868
Gross Unrealized Losses	(3,177)	(12,036)
Fair Value	1,292,042	1,137,675
Private-label
Investment securities available for sale
Amortized Cost	1,067	1,134
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(8)	(14)
Fair Value	1,059	1,121
Investment securities held to maturity
Amortized Cost	0
Fair Value	0
GNMA commercial mortgage-related securities
Investment securities available for sale
Amortized Cost	1,547,173	2,064,508
Gross Unrealized Gains	5	356
Gross Unrealized Losses	(33,901)	(35,966)
Fair Value	1,513,277	2,028,898
Investment securities held to maturity
Amortized Cost	546,098
Gross Unrealized Gains	9,546
Gross Unrealized Losses	(15,756)
Fair Value	539,888
Asset backed Securities
Investment securities available for sale
Amortized Cost	144,322
Gross Unrealized Gains	867
Gross Unrealized Losses	(13)
Fair Value	145,176
Investment securities held to maturity
Amortized Cost	0
Fair Value	0
Other securities (debt and equity)
Investment securities available for sale
Amortized Cost	4,719	4,718
Gross Unrealized Gains	127	105
Gross Unrealized Losses	(26)	(21)
Fair Value	4,820	4,802
FNMA / FHLMC \| Residential Related Securities
Investment securities available for sale
Amortized Cost	457,680	625,234
Gross Unrealized Gains	9,722	17,298
Gross Unrealized Losses	(2,634)	(2,602)
Fair Value	464,768	639,930
Investment securities held to maturity
Amortized Cost	40,995	37,697
Gross Unrealized Gains	398	439
Gross Unrealized Losses	(489)	(693)
Fair Value	40,904	37,443
GNMA \| Residential Related Securities
Investment securities available for sale
Amortized Cost	1,944,453	2,028,301
Gross Unrealized Gains	275	1,372
Gross Unrealized Losses	(31,378)	(25,198)
Fair Value	1,913,350	2,004,475
Investment securities held to maturity
Amortized Cost	414,440	89,996
Gross Unrealized Gains	2,700	216
Gross Unrealized Losses	(6,400)	(656)
Fair Value	$ 410,740	$ 89,556

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

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