Investments - Contractual Maturities of Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Cost
Due in one year or less	$ 184,819
Due after one year through five years	7,175
Due after ten through fifteen years	98
Cost	192,092
Preferred stock	265
Restricted investments	7,072
Amortized Cost	199,429	$ 646,273
Fair Value
Due in one year or less	184,740
Due after one year through five years	6,205
Due after ten through fifteen years	87
Fair Value	191,032
Preferred stock	208
Restricted investments	7,072
Available-for-sale Securities	$ 198,312	$ 644,818