Investment Securities, AFS and HTM Contractual Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 2,000
Due after one year through five years	2,000
Due after five years through ten years	0
Due after ten years	0
Total debt securities	4,000
Fair Value
Due in one year or less	1,999
Due after one year through five years	2,000
Due after five years through ten years	0
Due after ten years	0
Total debt securities	3,999
Investment securities available for sale, amortized cost	4,043,470	$ 4,100,417
Investment securities available for sale, at fair value	$ 3,948,510	4,043,446
Ratio of Fair Value to Amortized Cost	97.70%
Amortized Cost
Due in one year or less	$ 48,865
Due after one year through five years	187,828
Due after five years through ten years	409,682
Due after ten years	1,144,309
Total debt securities	1,790,683
Amortized Cost	2,740,511	2,282,853
Fair Value
Due in one year or less	49,128
Due after one year through five years	186,797
Due after five years through ten years	406,702
Due after ten years	1,141,032
Total debt securities	1,783,659
Total debt securities	$ 2,710,271	2,283,574
Ratio of Fair Value to Amortized Cost	98.90%
Private-label
Fair Value
Investment securities available for sale, amortized cost	$ 1,007	1,067
Investment securities available for sale, at fair value	1,003	1,059
Amortized Cost
Amortized Cost	0
Fair Value
Total debt securities	0
GNMA commercial mortgage-related securities
Fair Value
Investment securities available for sale, amortized cost	1,273,309	1,547,173
Investment securities available for sale, at fair value	1,220,797	1,513,277
Amortized Cost
Amortized Cost	505,434	546,098
Fair Value
Total debt securities	490,414	539,888
FFELP asset backed securities
Fair Value
Investment securities available for sale, amortized cost	297,347
Investment securities available for sale, at fair value	297,360
Amortized Cost
Amortized Cost	0
Fair Value
Total debt securities	0
Equity securities
Fair Value
Investment securities available for sale, amortized cost	1,568	1,519
Investment securities available for sale, at fair value	1,568	1,632
Amortized Cost
Amortized Cost	0
Fair Value
Total debt securities	0
FNMA / FHLMC \| Residential Related Securities
Fair Value
Investment securities available for sale, amortized cost	296,296	457,680
Investment securities available for sale, at fair value	295,252	464,768
Amortized Cost
Amortized Cost	92,788	40,995
Fair Value
Total debt securities	91,162	40,904
GNMA \| Residential Related Securities
Fair Value
Investment securities available for sale, amortized cost	2,169,943	1,944,453
Investment securities available for sale, at fair value	2,128,531	1,913,350
Amortized Cost
Amortized Cost	351,606	414,440
Fair Value
Total debt securities	$ 345,035	$ 410,740

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