Investment Securities - By Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized Cost
Due in One Year or Less	$ 105,997
Due after One Year through Five Years	396,025
Due after Five Years through Ten Years	908,710
Due after Ten Years	2,467,933
Total	3,878,665	$ 3,846,733
Fair Value
Due in One Year or Less	106,203
Due after One Year through Five Years	401,860
Due after Five Years through Ten Years	912,001
Due after Ten Years	2,437,273
Total	3,857,337	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	21,931
Amortized Cost	21,931	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	23,004
Total	23,004	25,013
Amortized Cost
Due in One Year or Less	105,997
Due after One Year through Five Years	396,025
Due after Five Years through Ten Years	908,710
Due after Ten Years	2,489,864
Total	3,900,596
Fair Value
Due in One Year or Less	106,203
Due after One Year through Five Years	401,860
Due after Five Years through Ten Years	912,001
Due after Ten Years	2,460,277
Total	3,880,341
U.S. Treasury
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	30,144
Due after Five Years through Ten Years	0
Due after Ten Years	0
Total	30,144	30,153
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	32,063
Due after Five Years through Ten Years	0
Due after Ten Years	0
Total	32,063	32,533
Agency
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	322,038
Due after Five Years through Ten Years	251,286
Due after Ten Years	87,258
Total	660,582	666,702
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	325,584
Due after Five Years through Ten Years	252,084
Due after Ten Years	85,491
Total	663,159	690,386
Corporate
Amortized Cost
Due in One Year or Less	94,711
Due after One Year through Five Years	9,683
Due after Five Years through Ten Years	218,847
Due after Ten Years	37,376
Total	360,617	412,223
Fair Value
Due in One Year or Less	94,917
Due after One Year through Five Years	9,645
Due after Five Years through Ten Years	218,536
Due after Ten Years	34,736
Total	357,834	415,308
Municipal bonds
Amortized Cost
Due in One Year or Less	11,286
Due after One Year through Five Years	4,622
Due after Five Years through Ten Years	36,182
Due after Ten Years	1,380,079
Total	1,432,169	1,412,012
Fair Value
Due in One Year or Less	11,286
Due after One Year through Five Years	4,912
Due after Five Years through Ten Years	38,652
Due after Ten Years	1,367,212
Total	1,422,062	1,446,019
Collateralized mortgage obligations
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	27,394
Due after Five Years through Ten Years	211,472
Due after Ten Years	275,384
Total	514,250	513,259
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	27,373
Due after Five Years through Ten Years	207,662
Due after Ten Years	273,936
Total	508,971	513,366
Mortgage-backed securities
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	2,144
Due after Five Years through Ten Years	190,923
Due after Ten Years	687,836
Total	880,903	812,384
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	2,283
Due after Five Years through Ten Years	195,067
Due after Ten Years	675,898
Total	873,248	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	20,353
Amortized Cost	20,353	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	21,426
Total	21,426	$ 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	1,578
Amortized Cost	1,578
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	1,578
Total	$ 1,578