Investments - Contractual Maturities of Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Amortized Cost
Due in one year or less	$ 333,636
Due after one year through five years	5,269
Amortized Cost	338,905
Preferred stock	11,858
Amortized Cost	350,763	$ 638,333
Fair Value
Due in one year or less	333,845
Due after one year through five years	5,269
Fair Value	339,114
Preferred stock	11,895
Fair Value	$ 351,009	$ 636,878