Investment Securities, AFS and HTM Contractual Maturities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Amortized Cost
Available for Sale, Due in one year or less	$ 7,987
Available for Sale, Due after one year through five years	75,545
Available for Sale, Due after five years through ten years	415,467
Available for Sale, Due after ten years	37,565
Available for Sale, Total debt securities	536,564
Available for Sale, amortized cost	3,872,150	$ 3,028,399
Fair Value
Available for Sale, Due in one year or less	7,996
Available for Sale, Due after one year through five years	75,970
Available for Sale, Due after five years through ten years	430,634
Available for Sale, Due after ten years	41,396
Available for Sale, Total debt securities	555,997
Investment securities AFS, at fair value	$ 3,893,379	3,085,441
Available for Sale, Ratio of Fair Value to Amortized Cost	100.50%
Amortized Cost
Held to Maturity, Due in one year or less	$ 27,797
Held to Maturity, Due after one year through five years	39,029
Held to Maturity, Due after five years through ten years	181,911
Held to Maturity, Due after ten years	1,293,705
Held to Maturity, Total Debt Securities	1,542,442
Held to Maturity, Amortized Cost	1,929,783	1,879,005
Fair Value
Held to Maturity, Due in one year or less	27,982
Held to Maturity, Due after one year through five years	40,265
Held to Maturity, Due after five years through ten years	189,290
Held to Maturity, Due after ten years	1,387,099
Held to Maturity, Securities, Debt Securities	1,644,635
Held to Maturity, Total debt securities at fair value	$ 2,035,858	2,028,943
Held to Maturity, Ratio of Fair Value to Amortized Cost	105.50%
Residential Related Securities \| Private-label Securities [Member]
Amortized Cost
Available for Sale, amortized cost	$ 76,033
Fair Value
Investment securities AFS, at fair value	76,033
Amortized Cost
Held to Maturity, Amortized Cost	0
Fair Value
Held to Maturity, Total debt securities at fair value	0
Residential Related Securities \| FNMA/FHLMC
Amortized Cost
Available for Sale, amortized cost	2,377,335	1,448,806
Fair Value
Investment securities AFS, at fair value	2,377,408
Amortized Cost
Held to Maturity, Amortized Cost	37,318	54,599
Fair Value
Held to Maturity, Total debt securities at fair value	39,447	57,490
Residential Related Securities \| Government National Mortgage Association (GNMA) [Member]
Amortized Cost
Available for Sale, amortized cost	77,199	231,364
Fair Value
Investment securities AFS, at fair value	79,696
Amortized Cost
Held to Maturity, Amortized Cost	54,900	114,553
Fair Value
Held to Maturity, Total debt securities at fair value	57,228	118,813
Commercial mortgage-related securities \| FNMA/FHLMC
Amortized Cost
Available for Sale, amortized cost	357,837	19,654
Fair Value
Investment securities AFS, at fair value	353,175
Amortized Cost
Held to Maturity, Amortized Cost	170,739	11,211
Fair Value
Held to Maturity, Total debt securities at fair value	166,985	11,211
Commercial mortgage-related securities \| Government National Mortgage Association (GNMA) [Member]
Amortized Cost
Available for Sale, amortized cost	227,622	511,429
Fair Value
Investment securities AFS, at fair value	231,096
Amortized Cost
Held to Maturity, Amortized Cost	124,383	255,742
Fair Value
Held to Maturity, Total debt securities at fair value	127,562	264,960
Federal Family Education Loan Program (FFELP) Guaranteed Loans [Member]
Amortized Cost
Available for Sale, amortized cost	212,378	329,030
Fair Value
Investment securities AFS, at fair value	212,797
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	7,182	$ 8,637
Fair Value
Investment securities AFS, at fair value	7,179
Amortized Cost
Held to Maturity, Amortized Cost	0
Fair Value
Held to Maturity, Total debt securities at fair value	$ 0

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