Investment Securities - By Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Amortized Cost
Due in One Year or Less	$ 77,883
Due after One Year through Five Years	138,347
Due after Five Years through Ten Years	610,325
Due after Ten Years	1,427,929
Total	2,254,484	$ 1,338,260
Fair Value
Due in One Year or Less	78,001
Due after One Year through Five Years	143,703
Due after Five Years through Ten Years	644,223
Due after Ten Years	1,470,139
Total	2,336,066	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	32,557
Amortized Cost	32,557	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	34,179
Total	34,179	38,760
Amortized Cost
Due in One Year or Less	77,883
Due after One Year through Five Years	138,347
Due after Five Years through Ten Years	610,325
Due after Ten Years	1,460,486
Total	2,287,041
Fair Value
Due in One Year or Less	78,001
Due after One Year through Five Years	143,703
Due after Five Years through Ten Years	644,223
Due after Ten Years	1,504,318
Total	2,370,245
U.S. Treasury
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	20,147
Due after Five Years through Ten Years	10,023
Due after Ten Years	0
Total	30,170	60,457
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	21,577
Due after Five Years through Ten Years	11,293
Due after Ten Years	0
Total	32,870	63,555
Agency
Amortized Cost
Due in One Year or Less	1,000
Due after One Year through Five Years	40,577
Due after Five Years through Ten Years	203,033
Due after Ten Years	71,317
Total	315,927	240,348
Fair Value
Due in One Year or Less	1,013
Due after One Year through Five Years	43,830
Due after Five Years through Ten Years	218,452
Due after Ten Years	76,695
Total	339,990	246,358
Corporate
Amortized Cost
Due in One Year or Less	76,883
Due after One Year through Five Years	73,958
Due after Five Years through Ten Years	123,235
Due after Ten Years	0
Total	274,076	149,150
Fair Value
Due in One Year or Less	76,988
Due after One Year through Five Years	74,303
Due after Five Years through Ten Years	122,708
Due after Ten Years	0
Total	273,999	151,353
Municipal bonds
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	1,452
Due after Five Years through Ten Years	29,653
Due after Ten Years	408,667
Total	439,772	384,032
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	1,574
Due after Five Years through Ten Years	31,980
Due after Ten Years	427,781
Total	461,335	397,298
Collateralized mortgage obligations
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	0
Due after Five Years through Ten Years	52,686
Due after Ten Years	327,841
Total	380,527	9,869
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	0
Due after Five Years through Ten Years	53,250
Due after Ten Years	331,489
Total	384,739	9,984
Mortgage-backed securities
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	2,213
Due after Five Years through Ten Years	191,695
Due after Ten Years	620,104
Total	814,012	494,404
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	2,419
Due after Five Years through Ten Years	206,540
Due after Ten Years	634,174
Total	843,133	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	30,892
Amortized Cost	30,892	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	32,514
Total	32,514	$ 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	1,665
Amortized Cost	1,665
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	1,665
Total	$ 1,665