Investment Securities, AFS and HTM Contractual Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Amortized Cost
Available for Sale, Due in one year or less	$ 10,731
Available for Sale, Due after one year through five years	30,829
Available for Sale, Due after five years through ten years	324,501
Available for Sale, Due after ten years	129,134
Available for Sale, Total debt securities	495,195
Available for Sale, amortized cost	2,897,851	$ 3,251,950
Fair Value
Available for Sale, Due in one year or less	10,738
Available for Sale, Due after one year through five years	31,242
Available for Sale, Due after five years through ten years	334,499
Available for Sale, Due after ten years	135,157
Available for Sale, Total debt securities	511,636
Investment securities available for sale, at fair value	$ 2,928,787	3,262,586
Available for Sale, Ratio of Fair Value to Amortized Cost	101.10%
Amortized Cost
Held to Maturity, Due in one year or less	$ 32,555
Held to Maturity, Due after one year through five years	77,586
Held to Maturity, Due after five years through ten years	153,519
Held to Maturity, Due after ten years	1,163,883
Held to Maturity, Total Debt Securities	1,427,542
Held to Maturity, Amortized Cost	2,149,434	2,205,083
Fair Value
Held to Maturity, Due in one year or less	32,703
Held to Maturity, Due after one year through five years	78,683
Held to Maturity, Due after five years through ten years	158,843
Held to Maturity, Due after ten years	1,239,262
Held to Maturity, Securities, Debt Securities	1,509,492
Held to Maturity, Total debt securities at fair value	$ 2,247,553	2,276,465
Held to Maturity, Ratio of Fair Value to Amortized Cost	104.60%
U.S. government SBA agency securities
Amortized Cost
Available for Sale, amortized cost	$ 10,241
Fair Value
Investment securities available for sale, at fair value	10,238
Amortized Cost
Held to Maturity, Amortized Cost	0
Fair Value
Held to Maturity, Total debt securities at fair value	0
FFELP asset backed securities
Amortized Cost
Available for Sale, amortized cost	360,658	270,178
Fair Value
Investment securities available for sale, at fair value	341,424	263,693
Amortized Cost
Held to Maturity, Amortized Cost	0
Fair Value
Held to Maturity, Total debt securities at fair value	0
FNMA/FHLMC \| Residential Related Securities
Amortized Cost
Available for Sale, amortized cost	103,576	131,158
Fair Value
Investment securities available for sale, at fair value	104,485	132,660
Amortized Cost
Held to Maturity, Amortized Cost	78,125	81,676
Fair Value
Held to Maturity, Total debt securities at fair value	81,610	83,420
FNMA/FHLMC \| Commercial mortgage-related securities
Amortized Cost
Available for Sale, amortized cost	19,861	19,929
Fair Value
Investment securities available for sale, at fair value	22,773	21,728
Amortized Cost
Held to Maturity, Amortized Cost	0
Fair Value
Held to Maturity, Total debt securities at fair value	0
GNMA \| Residential Related Securities
Amortized Cost
Available for Sale, amortized cost	799,983	982,941
Fair Value
Investment securities available for sale, at fair value	823,787	985,139
Amortized Cost
Held to Maturity, Amortized Cost	245,675	269,523
Fair Value
Held to Maturity, Total debt securities at fair value	253,799	270,296
GNMA \| Commercial mortgage-related securities
Amortized Cost
Available for Sale, amortized cost	1,108,336	1,314,836
Fair Value
Investment securities available for sale, at fair value	1,114,443	1,310,207
Amortized Cost
Held to Maturity, Amortized Cost	398,092	434,317
Fair Value
Held to Maturity, Total debt securities at fair value	$ 402,653	$ 434,503

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