Investment Securities, AFS and HTM Expected Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amortized Cost
Due in one year or less	$ 7,385
Due after one year through five years	83,293
Due after five years through ten years	350,687
Due after ten years	38,113
Total debt securities	479,478
Investment securities available for sale, amortized cost	3,028,399	$ 3,251,950
Fair Value
Due in one year or less	7,394
Due after one year through five years	84,112
Due after five years through ten years	371,076
Due after ten years	42,650
Total debt securities, AFS Fair Value	505,231
Investment securities AFS, at fair value	$ 3,085,441	3,262,586
Ratio of Fair Value to Amortized Cost	101.90%
Amortized Cost
Due in one year or less	$ 29,665
Due after one year through five years	54,313
Due after five years through ten years	187,853
Due after ten years	1,171,068
Amortized Cost	1,879,005	2,205,083
Fair Value
Due in one year or less	29,938
Due after one year through five years	56,196
Due after five years through ten years	197,850
Due after ten years	1,292,485
Debt Securities, Held-to-maturity, Fair Value	$ 2,028,943	2,276,465
Ratio of Fair Value to Amortized Cost	108.00%
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Amortized Cost	$ 1,442,900
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Fair Value	1,576,469
FFELP asset backed securities
Amortized Cost
Investment securities available for sale, amortized cost	329,030	270,178
Fair Value
Investment securities AFS, at fair value	327,189	263,693
Amortized Cost
Amortized Cost	0
Fair Value
Debt Securities, Held-to-maturity, Fair Value	0
SBA
Amortized Cost
Investment securities available for sale, amortized cost	8,637
Fair Value
Investment securities AFS, at fair value	8,584
Amortized Cost
Amortized Cost	0
Fair Value
Debt Securities, Held-to-maturity, Fair Value	0
FNMA/FHLMC [Member] \| Residential Related Securities
Amortized Cost
Investment securities available for sale, amortized cost	1,448,806	131,158
Fair Value
Investment securities AFS, at fair value	1,461,241	132,660
Amortized Cost
Amortized Cost	54,599	81,676
Fair Value
Debt Securities, Held-to-maturity, Fair Value	57,490	83,420
FNMA/FHLMC [Member] \| Commercial mortgage-related securities
Amortized Cost
Investment securities available for sale, amortized cost	19,654	19,929
Fair Value
Investment securities AFS, at fair value	22,904	21,728
Amortized Cost
Amortized Cost	11,211
Fair Value
Debt Securities, Held-to-maturity, Fair Value	11,211
GNMA \| Residential Related Securities
Amortized Cost
Investment securities available for sale, amortized cost	231,364	982,941
Fair Value
Investment securities AFS, at fair value	235,537	985,139
Amortized Cost
Amortized Cost	114,553	269,523
Fair Value
Debt Securities, Held-to-maturity, Fair Value	118,813	270,296
GNMA \| Commercial mortgage-related securities
Amortized Cost
Investment securities available for sale, amortized cost	511,429	1,314,836
Fair Value
Investment securities AFS, at fair value	524,756	1,310,207
Amortized Cost
Amortized Cost	255,742	434,317
Fair Value
Debt Securities, Held-to-maturity, Fair Value	$ 264,960	$ 434,503

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