Investment Securities - Contractual maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Amortized Cost
Due in One Year or Less	$ 1,499
Due after One Year through Five Years	64,910
Due after Five Years through Ten Years	561,851
Due after Ten Years	710,000
Amortized Cost	1,338,260	$ 1,111,168
Fair Value
Due in One Year or Less	1,514
Due after One Year through Five Years	67,052
Due after Five Years through Ten Years	572,589
Due after Ten Years	727,229
Total	1,368,384	1,103,222
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	908
Due after Five Years through Ten Years	0
Due after Ten Years	36,930
Amortized Cost	37,838	45,210
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	942
Due after Five Years through Ten Years	0
Due after Ten Years	37,818
Total	38,760	44,672
Amortized Cost
Due in One Year or Less	1,499
Due after One Year through Five Years	65,818
Due after Five Years through Ten Years	561,851
Due after Ten Years	746,930
Total	1,376,098
Fair Value
Due in One Year or Less	1,514
Due after One Year through Five Years	67,994
Due after Five Years through Ten Years	572,589
Due after Ten Years	765,047
Estimated Fair Value	1,407,144	1,147,894
U.S. Treasury
Amortized Cost
Due in One Year or Less	499
Due after One Year through Five Years	20,163
Due after Five Years through Ten Years	39,795
Due after Ten Years	0
Amortized Cost	60,457	59,688
Fair Value
Due in One Year or Less	500
Due after One Year through Five Years	20,586
Due after Five Years through Ten Years	42,469
Due after Ten Years	0
Total	63,555	60,912
Agency
Amortized Cost
Due in One Year or Less	1,000
Due after One Year through Five Years	40,647
Due after Five Years through Ten Years	166,244
Due after Ten Years	32,457
Amortized Cost	240,348	128,958
Fair Value
Due in One Year or Less	1,014
Due after One Year through Five Years	42,162
Due after Five Years through Ten Years	169,070
Due after Ten Years	34,112
Total	246,358	130,070
Corporate debt
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	0
Due after Five Years through Ten Years	135,407
Due after Ten Years	13,743
Amortized Cost	149,150	104,158
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	0
Due after Five Years through Ten Years	137,518
Due after Ten Years	13,835
Total	151,353	103,543
Municipal bonds
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	1,842
Due after Five Years through Ten Years	26,024
Due after Ten Years	356,166
Amortized Cost	384,032	238,914
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	1,952
Due after Five Years through Ten Years	26,996
Due after Ten Years	368,350
Total	397,298	238,630
Collateralized mortgage obligation: residential
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	0
Due after Five Years through Ten Years	610
Due after Ten Years	9,259
Amortized Cost	9,869	24,699
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	0
Due after Five Years through Ten Years	603
Due after Ten Years	9,381
Total	9,984	24,338
Mortgage-backed securities: residential
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	2,258
Due after Five Years through Ten Years	193,771
Due after Ten Years	298,375
Amortized Cost	494,404	554,751
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	2,352
Due after Five Years through Ten Years	195,933
Due after Ten Years	301,551
Total	499,836	545,729
Amortized Cost
Due in One Year or Less	0
Due after One Year through Five Years	908
Due after Five Years through Ten Years	0
Due after Ten Years	35,206
Amortized Cost	36,114	43,381
Fair Value
Due in One Year or Less	0
Due after One Year through Five Years	942
Due after Five Years through Ten Years	0
Due after Ten Years	36,094
Total	37,036	$ 42,843
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	1,724
Amortized Cost	1,724
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	1,724
Total	$ 1,724