Investment Securities - Amortized Cost and Fair Value by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amortized Cost
Due in One Year or Less	$ 156,044
Due after One Year through Five Years	417,881
Due after Five Years through Ten Years	894,968
Due after Ten Years	2,377,840
Amortized Cost	3,846,733	$ 1,338,260
Fair Value
Due in One Year or Less	156,996
Due after One Year through Five Years	428,702
Due after Five Years through Ten Years	923,253
Due after Ten Years	2,422,164
Total	3,931,115	1,368,384
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	23,732
Amortized Cost	23,732	37,838
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	25,013
Total	25,013	38,760
Amortized Cost
Due in One Year or Less	156,044
Due after One Year through Five Years	417,881
Due after Five Years through Ten Years	894,968
Due after Ten Years	2,401,572
Total	3,870,465
Fair Value
Due in One Year or Less	156,996
Due after One Year through Five Years	428,702
Due after Five Years through Ten Years	923,253
Due after Ten Years	2,447,177
Estimated Fair Value	3,956,128	1,407,144
Treasury
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	30,153
Due after Five Years through Ten Years	0
Due after Ten Years	0
Amortized Cost	30,153	60,457
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	32,533
Due after Five Years through Ten Years	0
Due after Ten Years	0
Total	32,533	63,555
Agency
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	357,471
Due after Five Years through Ten Years	216,904
Due after Ten Years	92,327
Amortized Cost	666,702	240,348
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	365,530
Due after Five Years through Ten Years	227,338
Due after Ten Years	97,518
Total	690,386	246,358
Corporate
Amortized Cost
Due in One Year or Less	146,122
Due after One Year through Five Years	9,696
Due after Five Years through Ten Years	218,964
Due after Ten Years	37,441
Amortized Cost	412,223	149,150
Fair Value
Due in One Year or Less	146,471
Due after One Year through Five Years	9,714
Due after Five Years through Ten Years	221,753
Due after Ten Years	37,370
Total	415,308	151,353
Municipal bonds
Amortized Cost
Due in One Year or Less	9,922
Due after One Year through Five Years	3,456
Due after Five Years through Ten Years	34,623
Due after Ten Years	1,364,011
Amortized Cost	1,412,012	384,032
Fair Value
Due in One Year or Less	10,525
Due after One Year through Five Years	3,743
Due after Five Years through Ten Years	37,721
Due after Ten Years	1,394,030
Total	1,446,019	397,298
Collateralized mortgage obligations
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	14,938
Due after Five Years through Ten Years	219,553
Due after Ten Years	278,768
Amortized Cost	513,259	9,869
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	14,869
Due after Five Years through Ten Years	219,509
Due after Ten Years	278,988
Total	513,366	9,984
Mortgage-backed securities
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	2,167
Due after Five Years through Ten Years	204,924
Due after Ten Years	605,293
Amortized Cost	812,384	494,404
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	2,313
Due after Five Years through Ten Years	216,932
Due after Ten Years	614,258
Total	833,503	499,836
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	22,124
Amortized Cost	22,124	36,114
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	23,405
Total	23,405	$ 37,036
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	1,608
Amortized Cost	1,608
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	1,608
Total	$ 1,608