Investment Securities, AFS and HTM Contractual Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized Cost
Available for Sale, Due in one year or less	$ 2,002
Available for Sale, Due after one year through five years	2,200
Available for Sale, Due after five years through ten years	0
Available for Sale, Due after ten years	0
Available for Sale, Total debt securities	4,202
Investment securities available for sale, amortized cost	4,584,763	$ 4,100,417
Fair Value
Available for Sale, Due in one year or less	1,995
Available for Sale, Due after one year through five years	2,188
Available for Sale, Due after five years through ten years	0
Available for Sale, Due after ten years	0
Available for Sale, Total debt securities	4,183
Investment securities available for sale, at fair value	$ 4,485,875	4,043,446
Ratio of Fair Value to Amortized Cost	97.80%
Amortized Cost
Held to Maturity, Due in one year or less	$ 44,019
Held to Maturity, Due after one year through five years	221,411
Held to Maturity, Due after five years through ten years	346,521
Held to Maturity, Due after ten years	812,974
Held to Maturity, Debt Securities, Amortized Cost Basis	1,424,925
Held to Maturity Amortized Cost	2,443,203	2,282,853
Fair Value
Held to Maturity, Due in one year or less	44,261
Held to Maturity, Due after one year through five years	219,411
Held to Maturity, Due after five years through ten years	341,147
Held to Maturity, Due after ten years	807,638
Held to Maturity, Securities, Debt Securities	1,412,456
Held to Maturity, Total debt securities	$ 2,407,197	2,283,574
Ratio of Fair Value to Amortized Cost	98.50%
Private Label
Amortized Cost
Investment securities available for sale, amortized cost	$ 1,051	1,067
Fair Value
Investment securities available for sale, at fair value	1,044	1,059
Amortized Cost
Held to Maturity Amortized Cost	0
Fair Value
Held to Maturity, Total debt securities	0
GNMA Commercial Mortgage-Related Securities
Amortized Cost
Investment securities available for sale, amortized cost	1,497,573	1,547,173
Fair Value
Investment securities available for sale, at fair value	1,448,369	1,513,277
Amortized Cost
Held to Maturity Amortized Cost	531,934
Fair Value
Held to Maturity, Total debt securities	519,248
Asset backed Securities
Amortized Cost
Investment securities available for sale, amortized cost	288,519
Fair Value
Investment securities available for sale, at fair value	290,300
Amortized Cost
Held to Maturity Amortized Cost	0
Fair Value
Held to Maturity, Total debt securities	0
Equity Securities [Member]
Amortized Cost
Investment securities available for sale, amortized cost	1,599	1,519
Fair Value
Investment securities available for sale, at fair value	1,599	1,632
Amortized Cost
Held to Maturity Amortized Cost	0
Fair Value
Held to Maturity, Total debt securities	0
FNMA and FHLMC \| Residential Related Securities
Amortized Cost
Investment securities available for sale, amortized cost	427,480	457,680
Fair Value
Investment securities available for sale, at fair value	430,073	464,768
Amortized Cost
Held to Maturity Amortized Cost	76,069	40,995
Fair Value
Held to Maturity, Total debt securities	74,631	40,904
GNMA \| Residential Related Securities
Amortized Cost
Investment securities available for sale, amortized cost	2,364,339	1,944,453
Fair Value
Investment securities available for sale, at fair value	2,310,306	1,913,350
Amortized Cost
Held to Maturity Amortized Cost	410,275	414,440
Fair Value
Held to Maturity, Total debt securities	$ 400,861	410,740
GNMA \| GNMA Commercial Mortgage-Related Securities
Amortized Cost
Held to Maturity Amortized Cost		546,098
Fair Value
Held to Maturity, Total debt securities		$ 539,888

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