Investments - Investments Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Available for sale, Amortized Cost
Due in one year or less	$ 27,529
Due after one year through five years	94,131
Due after five years through ten years	14,532
Due after ten years	2,900
Agency mortgage-backed securities	431,030
Available-for-sale Securities, Amortized Cost Basis	24,998
Amortized Cost	595,120	$ 463,143
Available for sale, Estimated Fair Value
Due in one year or less	27,563
Due after one year through five years	94,743
Due after five years through ten years	14,759
Due after ten years	2,620
Agency mortgage-backed securities	429,001
Available-for-sale, US Treasury Bills, Fair Value	24,999
Available for sale, fair value	593,685	460,797
Held to maturity, Amortized Cost
Due in one year or less	657
Due after one year through five years	7,056
Due after five years through ten years	6,523
Due after ten years	505
Agency mortgage-backed securities	64,495
Held-to-maturity, US Treasury Bills, Amortized Cost	0
Amortized Cost	79,236	80,463
Held to maturity, Estimated Fair Value
Due in one year or less	656
Due after one year through five years	7,043
Due after five years through ten years	6,479
Due after ten years	497
Agency mortgage-backed securities	63,942
Held-to-maturity, US Treasury Bills, Fair Value	0
Held to maturity, fair value	$ 78,617	$ 79,639