Investments - Contractual Maturities of Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Amortized Cost
Due in one year or less	$ 516,223
Due after one year through five years	102,366
Amortized Cost	618,589
Preferred stock	11,858
Amortized Cost	630,447	$ 638,333
Fair Value
Due in one year or less	516,461
Due after one year through five years	103,913
Fair Value	620,374
Preferred stock	11,982
Fair Value	$ 632,356	$ 636,878