Investment Securities - By Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
One Year or Less	$ 13,994
More than One Year to Five Years	55,816
More than Five Years to Ten Years	433,725
More than Ten Years	574,980
Total	1,078,515	$ 786,783
Fair Value
One Year or Less	13,990
More than One Year to Five Years	55,589
More than Five Years to Ten Years	426,710
More than Ten Years	558,588
Total	1,054,877	787,429
Amortized Cost
One Year or Less	0
More than One Year to Five Years	0
More than Five Years to Ten Years	933
More than Ten Years	45,452
Amortized Cost	46,385	18,291
Fair Value
One Year or Less	0
More than One Year to Five Years	0
More than Five Years to Ten Years	932
More than Ten Years	44,206
Total	45,138	18,082
Amortized Cost
One Year or Less	13,994
More than One Year to Five Years	55,816
More than Five Years to Ten Years	434,658
More than Ten Years	620,432
Total	1,124,900
Fair Value
One Year or Less	13,990
More than One Year to Five Years	55,589
More than Five Years to Ten Years	427,642
More than Ten Years	602,794
Total	1,100,015
U.S. Treasury
Amortized Cost
One Year or Less	0
More than One Year to Five Years	10,401
More than Five Years to Ten Years	49,258
More than Ten Years	0
Total	59,659
Fair Value
One Year or Less	0
More than One Year to Five Years	10,403
More than Five Years to Ten Years	48,940
More than Ten Years	0
Total	59,343
Agency
Amortized Cost
One Year or Less	10,977
More than One Year to Five Years	12,111
More than Five Years to Ten Years	68,902
More than Ten Years	21,638
Total	113,628	47,051
Fair Value
One Year or Less	10,977
More than One Year to Five Years	12,125
More than Five Years to Ten Years	68,379
More than Ten Years	21,290
Total	112,771	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	102,761
More than Ten Years	0
Total	102,761	78,155
Fair Value
One Year or Less	0
More than One Year to Five Years	0
More than Five Years to Ten Years	101,594
More than Ten Years	0
Total	101,594	79,546
Municipal bonds
Amortized Cost
One Year or Less	3,017
More than One Year to Five Years	31,511
More than Five Years to Ten Years	70,926
More than Ten Years	129,456
Total	234,910	228,929
Fair Value
One Year or Less	3,013
More than One Year to Five Years	31,334
More than Five Years to Ten Years	69,156
More than Ten Years	127,698
Total	231,201	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	872
More than Ten Years	25,025
Total	25,897	33,984
Fair Value
One Year or Less	0
More than One Year to Five Years	0
More than Five Years to Ten Years	871
More than Ten Years	24,335
Total	25,206	33,781
Mortgage-backed securities: residential
Amortized Cost
One Year or Less	0
More than One Year to Five Years	1,793
More than Five Years to Ten Years	141,006
More than Ten Years	398,861
Total	541,660	398,664
Fair Value
One Year or Less	0
More than One Year to Five Years	1,727
More than Five Years to Ten Years	137,770
More than Ten Years	385,265
Total	524,762	394,765
Amortized Cost
One Year or Less	0
More than One Year to Five Years	0
More than Five Years to Ten Years	933
More than Ten Years	44,354
Amortized Cost	45,287	17,153
Fair Value
One Year or Less	0
More than One Year to Five Years	0
More than Five Years to Ten Years	932
More than Ten Years	43,108
Total	44,040	$ 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	1,098
Amortized Cost	1,098
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	1,098
Total	$ 1,098