Investment Securities, AFS and HTM Securities Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investment securities available for sale:
Amortized Cost	$ 3,895,540	$ 4,041,902
Gross Unrealized Gains	4,013	3,649
Gross Unrealized Losses	(70,164)	(98,610)
Fair Value	3,829,388	3,946,941
Debt Securities, Held-to-maturity
Held to Maturity, Amortized Cost	2,846,689	2,740,511
Held-to-maturity, Gross Unrealized Gains	48,291	17,593
Held-to-maturity, Gross Unrealized Loss	(26,109)	(47,835)
Held-to-maturity, Fair Value	2,868,870	2,710,271
US Treasury Securities
Investment securities available for sale:
Amortized Cost		1,000
Gross Unrealized Gains		0
Gross Unrealized Losses		(1)
Fair Value		999
Debt Securities, Held-to-maturity
Held to Maturity, Amortized Cost	999
Held-to-maturity, Gross Unrealized Gains	8
Held-to-maturity, Gross Unrealized Loss	0
Held-to-maturity, Fair Value	1,007
Obligations of state and political subdivisions (municipal securities)
Debt Securities, Held-to-maturity
Held to Maturity, Amortized Cost	1,922,792	1,790,683
Held-to-maturity, Gross Unrealized Gains	39,043	8,255
Held-to-maturity, Gross Unrealized Loss	(1,349)	(15,279)
Held-to-maturity, Fair Value	1,960,487	1,783,659
Residential Related Securities \| FNMA/FHLMC
Investment securities available for sale:
Amortized Cost	242,921	296,296
Gross Unrealized Gains	1,607	2,466
Gross Unrealized Losses	(2,453)	(3,510)
Fair Value	242,074	295,252
Debt Securities, Held-to-maturity
Held to Maturity, Amortized Cost	91,003	92,788
Held-to-maturity, Gross Unrealized Gains	413	169
Held-to-maturity, Gross Unrealized Loss	(601)	(1,795)
Held-to-maturity, Fair Value	90,815	91,162
Residential Related Securities \| GNMA
Investment securities available for sale:
Amortized Cost	2,060,377	2,169,943
Gross Unrealized Gains	727	473
Gross Unrealized Losses	(23,004)	(41,885)
Fair Value	2,038,100	2,128,531
Debt Securities, Held-to-maturity
Held to Maturity, Amortized Cost	339,742	351,606
Held-to-maturity, Gross Unrealized Gains	1,714	1,611
Held-to-maturity, Gross Unrealized Loss	(3,314)	(8,181)
Held-to-maturity, Fair Value	338,141	345,035
Private-label
Investment securities available for sale:
Amortized Cost	877	1,007
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	(4)
Fair Value	878	1,003
Debt Securities, Held-to-maturity
Held to Maturity, Amortized Cost	0
Held-to-maturity, Fair Value	0
Commercial mortgage-related securities \| FNMA/FHLMC
Investment securities available for sale:
Amortized Cost	20,309
Gross Unrealized Gains	346
Gross Unrealized Losses	0
Fair Value	20,655
Debt Securities, Held-to-maturity
Held to Maturity, Amortized Cost	0
Held-to-maturity, Fair Value	0
Commercial mortgage-related securities \| GNMA
Investment securities available for sale:
Amortized Cost	1,291,892	1,273,309
Gross Unrealized Gains	402	0
Gross Unrealized Losses	(44,512)	(52,512)
Fair Value	1,247,782	1,220,797
Debt Securities, Held-to-maturity
Held to Maturity, Amortized Cost	492,154	505,434
Held-to-maturity, Gross Unrealized Gains	7,112	7,559
Held-to-maturity, Gross Unrealized Loss	(20,846)	(22,579)
Held-to-maturity, Fair Value	478,421	490,414
FFELP asset backed securities
Investment securities available for sale:
Amortized Cost	276,164	297,347
Gross Unrealized Gains	931	711
Gross Unrealized Losses	(195)	(698)
Fair Value	276,899	297,360
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	3,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	3,000	3,000
Investment Securities Class
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Securities	$ 3,600,000	$ 3,000,000

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