Investment Securities - By Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Amortized Cost
Due in One Year or Less	$ 170,870
Due after One Year through Five Years	330,075
Due after Five Years through Ten Years	761,169
Due after Ten Years	2,274,646
Total	3,536,760	$ 1,338,260
Fair Value
Due in One Year or Less	171,689
Due after One Year through Five Years	338,789
Due after Five Years through Ten Years	793,777
Due after Ten Years	2,296,476
Total	3,600,731	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	27,980
Amortized Cost	27,980	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	29,399
Total	29,399	38,760
Amortized Cost
Due in One Year or Less	170,870
Due after One Year through Five Years	330,075
Due after Five Years through Ten Years	761,169
Due after Ten Years	2,302,626
Total	3,564,740
Fair Value
Due in One Year or Less	171,689
Due after One Year through Five Years	338,789
Due after Five Years through Ten Years	793,777
Due after Ten Years	2,325,875
Total	3,630,130
U.S. Treasury
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	30,161
Due after Five Years through Ten Years	0
Due after Ten Years	0
Total	30,161	60,457
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	32,739
Due after Five Years through Ten Years	0
Due after Ten Years	0
Total	32,739	63,555
Agency
Amortized Cost
Due in One Year or Less	1,000
Due after One Year through Five Years	266,255
Due after Five Years through Ten Years	234,070
Due after Ten Years	76,302
Total	577,627	240,348
Fair Value
Due in One Year or Less	1,006
Due after One Year through Five Years	272,144
Due after Five Years through Ten Years	247,806
Due after Ten Years	82,089
Total	603,045	246,358
Corporate
Amortized Cost
Due in One Year or Less	159,949
Due after One Year through Five Years	26,391
Due after Five Years through Ten Years	156,824
Due after Ten Years	54,506
Total	397,670	149,150
Fair Value
Due in One Year or Less	160,420
Due after One Year through Five Years	26,363
Due after Five Years through Ten Years	156,934
Due after Ten Years	53,819
Total	397,536	151,353
Municipal bonds
Amortized Cost
Due in One Year or Less	9,921
Due after One Year through Five Years	1,458
Due after Five Years through Ten Years	36,895
Due after Ten Years	1,257,115
Total	1,305,389	384,032
Fair Value
Due in One Year or Less	10,263
Due after One Year through Five Years	1,586
Due after Five Years through Ten Years	39,849
Due after Ten Years	1,262,210
Total	1,313,908	397,298
Collateralized mortgage obligations
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	3,620
Due after Five Years through Ten Years	135,863
Due after Ten Years	218,132
Total	357,615	9,869
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	3,622
Due after Five Years through Ten Years	136,445
Due after Ten Years	218,213
Total	358,280	9,984
Mortgage-backed securities
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	2,190
Due after Five Years through Ten Years	197,517
Due after Ten Years	668,591
Total	868,298	494,404
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	2,335
Due after Five Years through Ten Years	212,743
Due after Ten Years	680,145
Total	895,223	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	26,357
Amortized Cost	26,357	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	27,776
Total	27,776	$ 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	1,623
Amortized Cost	1,623
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	1,623
Total	$ 1,623