Investment Securities, AFS and HTM Contractual Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amortized Cost
Due in one year or less	$ 1,001
Due after one year through five years	4,703
Due after five years through ten years	0
Due after ten years	0
Total debt securities	5,704
Fair Value
Due in one year or less	1,001
Due after one year through five years	4,669
Due after five years through ten years	0
Due after ten years	0
Total debt securities	5,670
Investment securities available for sale, amortized cost	4,100,417	$ 4,724,895
Investment securities available for sale, at fair value	$ 4,043,446	4,680,226
Ratio of Fair Value to Amortized Cost	98.60%
Amortized Cost
Due in one year or less	$ 50,089
Due after one year through five years	226,053
Due after five years through ten years	322,413
Due after ten years	682,765
Held-to-maturity Debt Securities, Amortized Cost Basis	1,281,320
Amortized Cost	2,282,853	1,273,536
Fair Value
Due in one year or less	44,781
Due after one year through five years	229,939
Due after five years through ten years	324,380
Due after ten years	692,942
Held-to-maturity Securities, Debt Securities	1,292,042
Total debt securities	$ 2,283,574	1,264,674
Ratio of Fair Value to Amortized Cost	100.00%
Private-label
Fair Value
Investment securities available for sale, amortized cost	$ 1,067	1,134
Investment securities available for sale, at fair value	1,059	1,121
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	1,547,173	2,064,508
Investment securities available for sale, at fair value	1,513,277	2,028,898
Amortized Cost
Amortized Cost	546,098
Fair Value
Total debt securities	539,888
Asset-backed Securities, Securitized Loans and Receivables [Member]
Fair Value
Investment securities available for sale, amortized cost	144,322
Investment securities available for sale, at fair value	145,176
Amortized Cost
Amortized Cost	0
Fair Value
Total debt securities	0
Equity securities
Fair Value
Investment securities available for sale, amortized cost	18
Investment securities available for sale, at fair value	146
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	457,680	625,234
Investment securities available for sale, at fair value	464,768	639,930
Amortized Cost
Amortized Cost	40,995	37,697
Fair Value
Total debt securities	40,904	37,443
GNMA \| Residential Related Securities
Fair Value
Investment securities available for sale, amortized cost	1,944,453	2,028,301
Investment securities available for sale, at fair value	1,913,350	2,004,475
Amortized Cost
Amortized Cost	414,440	89,996
Fair Value
Total debt securities	$ 410,740	$ 89,556

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