Investment Securities, AFS and HTM Contractual Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized Cost
Available for Sale, Due in one year or less	$ 9,575
Available for Sale, Due after one year through five years	79,504
Available for Sale, Due after five years through ten years	357,498
Available for Sale, Due after ten years	37,624
Available for Sale, Total debt securities	484,201
Available for Sale, amortized cost	3,323,846	$ 3,028,399
Fair Value
Available for Sale, Due in one year or less	9,588
Available for Sale, Due after one year through five years	79,813
Available for Sale, Due after five years through ten years	373,906
Available for Sale, Due after ten years	41,472
Available for Sale, Total debt securities	504,778
Investment securities AFS, at fair value	$ 3,356,949	3,085,441
Available for Sale, Ratio of Fair Value to Amortized Cost	101.00%
Amortized Cost
Held to Maturity, Due in one year or less	$ 35,299
Held to Maturity, Due after one year through five years	47,810
Held to Maturity, Due after five years through ten years	179,382
Held to Maturity, Due after ten years	1,193,970
Held to Maturity, Total Debt Securities	1,456,461
Held to Maturity, Amortized Cost	1,857,150	1,879,005
Fair Value
Held to Maturity, Due in one year or less	35,587
Held to Maturity, Due after one year through five years	49,383
Held to Maturity, Due after five years through ten years	188,263
Held to Maturity, Due after ten years	1,289,186
Held to Maturity, Securities, Debt Securities	1,562,420
Held to Maturity, Total debt securities at fair value	$ 1,972,541	2,028,943
Held to Maturity, Ratio of Fair Value to Amortized Cost	106.20%
SBA
Amortized Cost
Available for Sale, amortized cost	$ 8,216	8,637
Fair Value
Investment securities AFS, at fair value	8,160
Amortized Cost
Held to Maturity, Amortized Cost	0
Fair Value
Held to Maturity, Total debt securities at fair value	0
FFELP
Amortized Cost
Available for Sale, amortized cost	325,812	329,030
Fair Value
Investment securities AFS, at fair value	325,933
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	1,910,644	1,448,806
Fair Value
Investment securities AFS, at fair value	1,910,954
Amortized Cost
Held to Maturity, Amortized Cost	47,480	54,599
Fair Value
Held to Maturity, Total debt securities at fair value	49,951	57,490
FNMA/FHLMC \| Commercial mortgage-related securities
Amortized Cost
Available for Sale, amortized cost	93,902	19,654
Fair Value
Investment securities AFS, at fair value	94,349
Amortized Cost
Held to Maturity, Amortized Cost	63,844	11,211
Fair Value
Held to Maturity, Total debt securities at fair value	61,863	11,211
GNMA \| Residential Related Securities
Amortized Cost
Available for Sale, amortized cost	119,913	231,364
Fair Value
Investment securities AFS, at fair value	123,365
Amortized Cost
Held to Maturity, Amortized Cost	87,717	114,553
Fair Value
Held to Maturity, Total debt securities at fair value	91,070	118,813
GNMA \| Commercial mortgage-related securities
Amortized Cost
Available for Sale, amortized cost	381,158	511,429
Fair Value
Investment securities AFS, at fair value	389,411
Amortized Cost
Held to Maturity, Amortized Cost	201,648	255,742
Fair Value
Held to Maturity, Total debt securities at fair value	$ 207,238	$ 264,960

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