Investment Securities, AFS and HTM Securities Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities, Amortized Cost Basis, Total	$ 3,872,150	$ 3,028,399
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	38,106	60,612
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(16,877)	(3,570)
Available-for-sale Securities	3,893,379	3,085,441
Investment securities HTM
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss	1,929,783	1,879,005
Held-to-maturity, Gross Unrealized Gains	114,095	149,938
Held-to-maturity, Gross Unrealized Loss	(8,019)	0
Held-to-maturity, Fair Value	2,035,858	2,028,943
Fair Value, Inputs, Level 2
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities	3,769,399	3,058,910
Investment securities HTM
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss	1,928,735	1,877,939
Held-to-maturity, Fair Value	2,034,803	2,027,852
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Other Debt securities
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities	2,999	3,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | US Government-sponsored Enterprises Debt Securities [Member]
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities	7,179	8,584
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Federal Family Education Loan Program (FFELP) Guaranteed Loans [Member]
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities	212,797	327,189
Fair Value, Inputs, Level 1
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities	123,981	26,531
Private-label Securities [Member] | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Residential Related Securities
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities	76,033	0
FNMA/FHLMC | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Residential Related Securities
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities	2,377,408	1,461,241
FNMA/FHLMC | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Commercial mortgage-related securities
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities	353,175	22,904
Government National Mortgage Association (GNMA) [Member] | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Residential Related Securities
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities	79,696	235,537
Government National Mortgage Association (GNMA) [Member] | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Commercial mortgage-related securities
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities	231,096	524,756
US Treasury Securities [Member]
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities, Amortized Cost Basis, Total	124,253	26,436
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	46	95
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(319)	0
Investment securities HTM
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss	1,000	999
Held-to-maturity, Gross Unrealized Gains	7	25
Held-to-maturity, Gross Unrealized Loss	0	0
Held-to-maturity, Fair Value	1,007	1,024
US Treasury Securities [Member] | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities		26,531
US Treasury Securities [Member] | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities	123,981	26,531
Agency Securities
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities, Amortized Cost Basis, Total	15,000	24,985
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	53
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(2)	0
Agency Securities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities	14,998	25,038
US States and Political Subdivisions Debt Securities [Member]
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities, Amortized Cost Basis, Total	394,310	425,057
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	19,709	25,605
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Investment securities HTM
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss	1,541,443	1,441,900
Held-to-maturity, Gross Unrealized Gains	106,181	133,544
Held-to-maturity, Gross Unrealized Loss	(3,996)	0
Held-to-maturity, Fair Value	1,643,628	1,575,445
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities	414,020	450,662
Residential Related Securities | Private-label Securities [Member]
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities, Amortized Cost Basis, Total	76,033
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0
Available-for-sale Securities	76,033
Investment securities HTM
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss	0
Held-to-maturity, Fair Value	0
Residential Related Securities | FNMA/FHLMC
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities, Amortized Cost Basis, Total	2,377,335	1,448,806
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	9,216	12,935
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(9,144)	(500)
Available-for-sale Securities	2,377,408
Investment securities HTM
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss	37,318	54,599
Held-to-maturity, Gross Unrealized Gains	2,129	2,891
Held-to-maturity, Gross Unrealized Loss	0	0
Held-to-maturity, Fair Value	39,447	57,490
Residential Related Securities | Government National Mortgage Association (GNMA) [Member]
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities, Amortized Cost Basis, Total	77,199	231,364
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2,497	4,176
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	(3)
Available-for-sale Securities	79,696
Investment securities HTM
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss	54,900	114,553
Held-to-maturity, Gross Unrealized Gains	2,328	4,260
Held-to-maturity, Gross Unrealized Loss	0	0
Held-to-maturity, Fair Value	57,228	118,813
Commercial mortgage-related securities | FNMA/FHLMC
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities, Amortized Cost Basis, Total	357,837	19,654
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2,230	3,250
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(6,892)	0
Available-for-sale Securities	353,175
Investment securities HTM
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss	170,739	11,211
Held-to-maturity, Gross Unrealized Gains	248	0
Held-to-maturity, Gross Unrealized Loss	(4,002)	0
Held-to-maturity, Fair Value	166,985	11,211
Commercial mortgage-related securities | Government National Mortgage Association (GNMA) [Member]
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities, Amortized Cost Basis, Total	227,622	511,429
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	3,474	13,327
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Available-for-sale Securities	231,096
Investment securities HTM
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss	124,383	255,742
Held-to-maturity, Gross Unrealized Gains	3,202	9,218
Held-to-maturity, Gross Unrealized Loss	(22)	0
Held-to-maturity, Fair Value	127,562	264,960
Federal Family Education Loan Program (FFELP) Guaranteed Loans [Member]
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities, Amortized Cost Basis, Total	212,378	329,030
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	890	1,172
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(471)	(3,013)
Available-for-sale Securities	212,797
Investment securities HTM
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss	0
Held-to-maturity, Fair Value	0
US Government-sponsored Enterprises Debt Securities [Member]
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities, Amortized Cost Basis, Total	7,182	8,637
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	44	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(48)	(53)
Available-for-sale Securities	7,179
Investment securities HTM
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss	0
Held-to-maturity, Fair Value	0
US Government-sponsored Enterprises Debt Securities [Member] | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities		8,584
Other Debt securities
Debt Securities, Available-for-sale [Abstract]
Available-for-sale Securities, Amortized Cost Basis, Total	3,000	3,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	$ (1)	$ 0