Investment Securities - By Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Amortized Cost
Due in One Year or Less	$ 10,021
Due after One Year through Five Years	460,645
Due after Five Years through Ten Years	1,567,613
Due after Ten Years	2,666,833
Amortized  Cost	4,705,112	$ 3,846,733
Fair Value
Due in One Year or Less	10,029
Due after One Year through Five Years	464,211
Due after Five Years through Ten Years	1,566,785
Due after Ten Years	2,668,790
Total	4,709,815	3,931,115
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	0
Due after Five Years through Ten Years	0
Due after Ten Years	170,587
Total	170,587	23,732
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	0
Due after Five Years through Ten Years	0
Due after Ten Years	169,154
Total	169,154	25,013
Amortized Cost
Due in One Year or Less	10,021
Due after One Year through Five Years	460,645
Due after Five Years through Ten Years	1,567,613
Due after Ten Years	2,837,420
Total	4,875,699
Fair Value
Due in One Year or Less	10,029
Due after One Year through Five Years	464,211
Due after Five Years through Ten Years	1,566,785
Due after Ten Years	2,837,944
Total	4,878,969
U.S. Treasury
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	30,126
Due after Five Years through Ten Years	47,677
Due after Ten Years	0
Amortized  Cost	77,803	30,153
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	31,734
Due after Five Years through Ten Years	47,608
Due after Ten Years	0
Total	79,342	32,533
Agency
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	352,023
Due after Five Years through Ten Years	135,567
Due after Ten Years	44,277
Amortized  Cost	531,867	666,702
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	353,184
Due after Five Years through Ten Years	134,426
Due after Ten Years	42,483
Total	530,093	690,386
Corporate
Amortized Cost
Due in One Year or Less	10,021
Due after One Year through Five Years	21,178
Due after Five Years through Ten Years	352,711
Due after Ten Years	0
Amortized  Cost	383,910	412,223
Fair Value
Due in One Year or Less	10,029
Due after One Year through Five Years	21,227
Due after Five Years through Ten Years	355,135
Due after Ten Years	0
Total	386,391	415,308
Municipal bonds
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	3,528
Due after Five Years through Ten Years	95,420
Due after Ten Years	1,138,975
Amortized  Cost	1,237,923	1,412,012
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	3,731
Due after Five Years through Ten Years	96,849
Due after Ten Years	1,155,286
Total	1,255,866	1,446,019
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	0
Due after Five Years through Ten Years	0
Due after Ten Years	154,399
Total	154,399
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	0
Due after Five Years through Ten Years	0
Due after Ten Years	152,163
Total	152,163
Collateralized mortgage obligations
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	41,692
Due after Five Years through Ten Years	226,958
Due after Ten Years	397,201
Amortized  Cost	665,851	513,259
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	41,631
Due after Five Years through Ten Years	223,612
Due after Ten Years	396,988
Total	662,231	513,366
Mortgage-backed securities
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	12,098
Due after Five Years through Ten Years	709,280
Due after Ten Years	1,086,380
Amortized  Cost	1,807,758	812,384
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	12,704
Due after Five Years through Ten Years	709,155
Due after Ten Years	1,074,033
Total	1,795,892	833,503
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	0
Due after Five Years through Ten Years	0
Due after Ten Years	14,656
Total	14,656	22,124
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	0
Due after Five Years through Ten Years	0
Due after Ten Years	15,459
Total	15,459	$ 23,405
Other
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	0
Due after Five Years through Ten Years	0
Due after Ten Years	1,532
Total	1,532
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	0
Due after Five Years through Ten Years	0
Due after Ten Years	1,532
Total	$ 1,532