Investment Securities, AFS and HTM Contractual Maturities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Amortized Cost
Available for Sale, Due in one year or less	$ 6,278
Available for Sale, Due after one year through five years	77,593
Available for Sale, Due after five years through ten years	419,367
Available for Sale, Due after ten years	37,595
Available for Sale, Total debt securities	540,833
Available for Sale, amortized cost	3,282,290	$ 3,028,399
Fair Value
Available for Sale, Due in one year or less	6,285
Available for Sale, Due after one year through five years	78,211
Available for Sale, Due after five years through ten years	436,807
Available for Sale, Due after ten years	41,829
Available for Sale, Total debt securities	563,132
Investment securities AFS, at fair value	$ 3,323,346	3,085,441
Available for Sale, Ratio of Fair Value to Amortized Cost	101.30%
Amortized Cost
Held to Maturity, Due in one year or less	$ 29,227
Held to Maturity, Due after one year through five years	42,396
Held to Maturity, Due after five years through ten years	175,687
Held to Maturity, Due after ten years	1,233,065
Held to Maturity, Total Debt Securities	1,480,375
Held to Maturity, Amortized Cost	1,799,893	1,879,005
Fair Value
Held to Maturity, Due in one year or less	29,460
Held to Maturity, Due after one year through five years	43,855
Held to Maturity, Due after five years through ten years	184,271
Held to Maturity, Due after ten years	1,344,520
Held to Maturity, Securities, Debt Securities	1,602,106
Held to Maturity, Total debt securities at fair value	$ 1,930,541	2,028,943
Held to Maturity, Ratio of Fair Value to Amortized Cost	107.30%
Residential Related Securities \| FNMA/FHLMC
Amortized Cost
Available for Sale, amortized cost	$ 2,049,513	1,448,806
Fair Value
Investment securities AFS, at fair value	2,057,909
Amortized Cost
Held to Maturity, Amortized Cost	41,505	54,599
Fair Value
Held to Maturity, Total debt securities at fair value	43,656	57,490
Residential Related Securities \| Government National Mortgage Association (GNMA) [Member]
Amortized Cost
Available for Sale, amortized cost	90,956	231,364
Fair Value
Investment securities AFS, at fair value	93,967
Amortized Cost
Held to Maturity, Amortized Cost	66,979	114,553
Fair Value
Held to Maturity, Total debt securities at fair value	69,597	118,813
Commercial mortgage-related securities \| FNMA/FHLMC
Amortized Cost
Available for Sale, amortized cost	93,693	19,654
Fair Value
Investment securities AFS, at fair value	95,708
Amortized Cost
Held to Maturity, Amortized Cost	63,668	11,211
Fair Value
Held to Maturity, Total debt securities at fair value	63,591	11,211
Commercial mortgage-related securities \| Government National Mortgage Association (GNMA) [Member]
Amortized Cost
Available for Sale, amortized cost	284,815	511,429
Fair Value
Investment securities AFS, at fair value	289,593
Amortized Cost
Held to Maturity, Amortized Cost	147,366	255,742
Fair Value
Held to Maturity, Total debt securities at fair value	151,591	264,960
Federal Family Education Loan Program (FFELP) Guaranteed Loans [Member]
Amortized Cost
Available for Sale, amortized cost	214,904	329,030
Fair Value
Investment securities AFS, at fair value	215,464
Amortized Cost
Held to Maturity, Amortized Cost	0
Fair Value
Held to Maturity, Total debt securities at fair value	0
US Government-sponsored Enterprises Debt Securities [Member]
Amortized Cost
Available for Sale, amortized cost	7,575	$ 8,637
Fair Value
Investment securities AFS, at fair value	7,573
Amortized Cost
Held to Maturity, Amortized Cost	0
Fair Value
Held to Maturity, Total debt securities at fair value	$ 0

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