Investment Securities - By Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Amortized Cost
Due in One Year or Less	$ 66,854
Due after One Year through Five Years	384,993
Due after Five Years through Ten Years	1,342,839
Due after Ten Years	2,655,663
Total	4,450,349	$ 3,846,733
Fair Value
Due in One Year or Less	66,904
Due after One Year through Five Years	390,106
Due after Five Years through Ten Years	1,352,508
Due after Ten Years	2,677,929
Total	4,487,447	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	18,933
Amortized Cost	18,933	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	19,820
Total	19,820	25,013
Amortized Cost
Due in One Year or Less	66,854
Due after One Year through Five Years	384,993
Due after Five Years through Ten Years	1,342,839
Due after Ten Years	2,674,596
Amortized Cost	4,469,282	3,870,465
Fair Value
Due in One Year or Less	66,904
Due after One Year through Five Years	390,106
Due after Five Years through Ten Years	1,352,508
Due after Ten Years	2,697,749
Estimated Fair Value	4,507,267	3,956,128
U.S. Treasury
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	30,135
Due after Five Years through Ten Years	79,712
Due after Ten Years	0
Total	109,847	30,153
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	31,913
Due after Five Years through Ten Years	79,895
Due after Ten Years	0
Total	111,808	32,533
Agency
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	312,054
Due after Five Years through Ten Years	184,783
Due after Ten Years	57,505
Total	554,342	666,702
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	314,952
Due after Five Years through Ten Years	184,414
Due after Ten Years	55,801
Total	555,167	690,386
Corporate
Amortized Cost
Due in One Year or Less	54,024
Due after One Year through Five Years	9,670
Due after Five Years through Ten Years	291,407
Due after Ten Years	0
Total	355,101	412,223
Fair Value
Due in One Year or Less	54,074
Due after One Year through Five Years	9,714
Due after Five Years through Ten Years	294,594
Due after Ten Years	0
Total	358,382	415,308
Municipal bonds
Amortized Cost
Due in One Year or Less	12,830
Due after One Year through Five Years	4,619
Due after Five Years through Ten Years	77,533
Due after Ten Years	1,253,364
Total	1,348,346	1,412,012
Fair Value
Due in One Year or Less	12,830
Due after One Year through Five Years	4,918
Due after Five Years through Ten Years	80,607
Due after Ten Years	1,281,118
Total	1,379,473	1,446,019
Collateralized mortgage obligations
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	26,394
Due after Five Years through Ten Years	229,519
Due after Ten Years	361,683
Total	617,596	513,259
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	26,357
Due after Five Years through Ten Years	226,997
Due after Ten Years	361,384
Total	614,738	513,366
Mortgage-backed securities
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	2,121
Due after Five Years through Ten Years	479,885
Due after Ten Years	983,111
Total	1,465,117	812,384
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	2,252
Due after Five Years through Ten Years	486,001
Due after Ten Years	979,626
Total	1,467,879	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	17,378
Amortized Cost	17,378	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	18,265
Total	18,265	$ 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	1,555
Amortized Cost	1,555
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	1,555
Total	$ 1,555