Investment Securities - Contractual maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Amortized Cost
One Year or Less	$ 6,263
More than One Year to Five Years	76,794
More than Five Years to Ten Years	461,612
More than Ten Years	566,499
Amortized Cost	1,111,168	$ 786,783
Fair Value
One Year or Less	6,255
More than One Year to Five Years	77,606
More than Five Years to Ten Years	461,134
More than Ten Years	558,227
Total	1,103,222	787,429
Amortized Cost
One Year or Less	0
More than One Year to Five Years	928
More than Five Years to Ten Years	0
More than Ten Years	44,282
Amortized Cost	45,210	18,291
Fair Value
One Year or Less	0
More than One Year to Five Years	942
More than Five Years to Ten Years	0
More than Ten Years	43,730
Total	44,672	18,082
Amortized Cost
One Year or Less	6,263
More than One Year to Five Years	77,722
More than Five Years to Ten Years	461,612
More than Ten Years	610,781
Total	1,156,378
Fair Value
One Year or Less	6,255
More than One Year to Five Years	78,548
More than Five Years to Ten Years	461,134
More than Ten Years	601,957
Estimated Fair Value	1,147,894	805,511
U.S. Treasury
Amortized Cost
One Year or Less	0
More than One Year to Five Years	10,407
More than Five Years to Ten Years	49,281
More than Ten Years	0
Amortized Cost	59,688
Fair Value
One Year or Less	0
More than One Year to Five Years	10,606
More than Five Years to Ten Years	50,306
More than Ten Years	0
Total	60,912
Agency
Amortized Cost
One Year or Less	992
More than One Year to Five Years	35,103
More than Five Years to Ten Years	74,149
More than Ten Years	18,714
Amortized Cost	128,958	47,051
Fair Value
One Year or Less	991
More than One Year to Five Years	35,769
More than Five Years to Ten Years	74,926
More than Ten Years	18,384
Total	130,070	47,209
Corporate debt
Amortized Cost
One Year or Less	0
More than One Year to Five Years	0
More than Five Years to Ten Years	104,158
More than Ten Years	0
Amortized Cost	104,158	78,155
Fair Value
One Year or Less	0
More than One Year to Five Years	0
More than Five Years to Ten Years	103,543
More than Ten Years	0
Total	103,543	79,546
Municipal bonds
Amortized Cost
One Year or Less	5,271
More than One Year to Five Years	29,715
More than Five Years to Ten Years	70,354
More than Ten Years	133,574
Amortized Cost	238,914	228,929
Fair Value
One Year or Less	5,264
More than One Year to Five Years	29,704
More than Five Years to Ten Years	69,581
More than Ten Years	134,081
Total	238,630	232,128
Collateralized mortgage obligation: residential
Amortized Cost
One Year or Less	0
More than One Year to Five Years	0
More than Five Years to Ten Years	815
More than Ten Years	23,884
Amortized Cost	24,699	33,984
Fair Value
One Year or Less	0
More than One Year to Five Years	0
More than Five Years to Ten Years	814
More than Ten Years	23,524
Total	24,338	33,781
Mortgage-backed securities: residential
Amortized Cost
One Year or Less	0
More than One Year to Five Years	1,569
More than Five Years to Ten Years	162,855
More than Ten Years	390,327
Amortized Cost	554,751	398,664
Fair Value
One Year or Less	0
More than One Year to Five Years	1,527
More than Five Years to Ten Years	161,964
More than Ten Years	382,238
Total	545,729	394,765
Amortized Cost
One Year or Less	0
More than One Year to Five Years	928
More than Five Years to Ten Years	0
More than Ten Years	42,453
Amortized Cost	43,381	17,153
Fair Value
One Year or Less	0
More than One Year to Five Years	942
More than Five Years to Ten Years	0
More than Ten Years	41,901
Total	42,843	$ 16,944
Other
Amortized Cost
One Year or Less	0
More than One Year to Five Years	0
More than Five Years to Ten Years	0
More than Ten Years	1,829
Amortized Cost	1,829
Fair Value
One Year or Less	0
More than One Year to Five Years	0
More than Five Years to Ten Years	0
More than Ten Years	1,829
Total	$ 1,829