Investment Securities - Contractual maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amortized Cost
One Year or Less	$ 6,715
More than One Year to Five Years	35,037
More than Five Years to Ten Years	232,766
More than Ten Years	512,265
Amortized Cost	786,783	$ 385,662
Fair Value
One Year or Less	6,704
More than One Year to Five Years	35,035
More than Five Years to Ten Years	234,107
More than Ten Years	511,583
Total	787,429	380,963
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	18,291
Amortized Cost	18,291	8,565
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	18,082
Total	18,082	8,461
Amortized Cost
One Year or Less	6,715
More than One Year to Five Years	35,037
More than Five Years to Ten Years	232,766
More than Ten Years	530,556
Total	805,074
Fair Value
One Year or Less	6,704
More than One Year to Five Years	35,035
More than Five Years to Ten Years	234,107
More than Ten Years	529,665
Total	805,511
Agency
Amortized Cost
One Year or Less	0
More than One Year to Five Years	0
More than Five Years to Ten Years	15,156
More than Ten Years	31,895
Amortized Cost	47,051
Fair Value
One Year or Less	0
More than One Year to Five Years	0
More than Five Years to Ten Years	15,164
More than Ten Years	32,045
Total	47,209
Corporate
Amortized Cost
One Year or Less	0
More than One Year to Five Years	0
More than Five Years to Ten Years	78,155
More than Ten Years	0
Amortized Cost	78,155	37,475
Fair Value
One Year or Less	0
More than One Year to Five Years	0
More than Five Years to Ten Years	79,546
More than Ten Years	0
Total	79,546	37,642
Municipal bonds
Amortized Cost
One Year or Less	4,124
More than One Year to Five Years	32,390
More than Five Years to Ten Years	72,845
More than Ten Years	119,570
Amortized Cost	228,929	120,155
Fair Value
One Year or Less	4,121
More than One Year to Five Years	32,424
More than Five Years to Ten Years	73,312
More than Ten Years	122,271
Total	232,128	118,803
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	1,069
More than Ten Years	32,915
Amortized Cost	33,984	31,536
Fair Value
One Year or Less	0
More than One Year to Five Years	0
More than Five Years to Ten Years	1,071
More than Ten Years	32,710
Total	33,781	31,388
Mortgage-backed securities: residential
Amortized Cost
One Year or Less	2,591
More than One Year to Five Years	2,647
More than Five Years to Ten Years	65,541
More than Ten Years	327,885
Amortized Cost	398,664	196,496
Fair Value
One Year or Less	2,583
More than One Year to Five Years	2,611
More than Five Years to Ten Years	65,014
More than Ten Years	324,557
Total	394,765	193,130
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	17,153
Amortized Cost	17,153	7,375
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	16,944
Total	16,944	$ 7,271
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	1,138
Amortized Cost	1,138
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	1,138
Total	$ 1,138