Investments - Contractual Maturities of Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Due in one year or less, Cost	$ 659,930
Due one year through five years, Cost	771,762
Due after five years through ten years, Cost	30,744
Due after ten years, Cost	2,500
Amortized Cost	1,464,936	$ 1,025,951
Due in one year or less, Estimated Fair Value	659,729
Due one year through five years, Estimated Fair Value	770,930
Due after five years through ten years, Estimated Fair Value	30,808
Due after ten years, Estimated Fair Value	2,355
Total, Estimated Fair Value	$ 1,463,822