Investments - Investments Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Available for sale, Amortized Cost
Due in one year or less	$ 52,681
Due after one year through five years	70,311
Due after five years through ten years	9,009
Due after ten years	3,531
Agency mortgage-backed securities	333,549
Amortized Cost	469,081	$ 450,915
Available for sale, Estimated Fair Value
Due in one year or less	53,030
Due after one year through five years	72,722
Due after five years through ten years	9,607
Due after ten years	3,292
Agency mortgage-backed securities	339,863
Available for sale, fair value	478,514	451,770
Held to maturity, Amortized Cost
Due in one year or less	0
Due after one year through five years	7,365
Due after five years through ten years	7,430
Due after ten years	0
Agency mortgage-backed securities	27,719
Amortized Cost	42,514	43,714
Held to maturity, Estimated Fair Value
Due in one year or less	0
Due after one year through five years	7,569
Due after five years through ten years	7,694
Due after ten years	0
Agency mortgage-backed securities	28,574
Held to maturity, fair value	$ 43,837	$ 43,441