Investments - Investments Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available for sale, Amortized Cost
Due in one year or less	$ 3,060
Due after one year through five years	110,910
Due after five years through ten years	14,573
Due after ten years	5,516
Agency mortgage-backed securities	513,082
Amortized Cost	647,141	$ 463,143
Available for sale, Estimated Fair Value
Due in one year or less	3,076
Due after one year through five years	110,480
Due after five years through ten years	14,980
Due after ten years	5,330
Agency mortgage-backed securities	507,516
Securities available for sale	641,382	460,797
Held to maturity, Amortized Cost
Due in one year or less	0
Due after one year through five years	186
Due after five years through ten years	12,977
Due after ten years	868
Agency mortgage-backed securities	59,718
Amortized Cost	73,749	80,463
Held to maturity, Estimated Fair Value
Due in one year or less	0
Due after one year through five years	195
Due after five years through ten years	12,981
Due after ten years	878
Agency mortgage-backed securities	59,404
Held to maturity, fair value	$ 73,458	$ 79,639