Investments - Investments Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available for sale, Amortized Cost
Due in one year or less	$ 2,404
Due after one year through five years	120,488
Due after five years through ten years	14,514
Due after ten years	5,050
Agency mortgage-backed securities	524,738
Amortized Cost	667,194	$ 647,141
Available for sale, Estimated Fair Value
Due in one year or less	2,419
Due after one year through five years	118,875
Due after five years through ten years	14,647
Due after ten years	4,946
Agency mortgage-backed securities	511,385
Available for sale, fair value	652,272	641,382
Held to maturity, Amortized Cost
Due in one year or less	0
Due after one year through five years	865
Due after five years through ten years	10,820
Due after ten years	867
Agency mortgage-backed securities	58,027
Amortized Cost	70,579	73,749
Held to maturity, Estimated Fair Value
Due in one year or less	0
Due after one year through five years	858
Due after five years through ten years	10,613
Due after ten years	847
Agency mortgage-backed securities	56,448
Held to maturity, fair value	$ 68,766	$ 73,458