Contractual Maturities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013
Contractual maturities of investments
Due in one year or less, Amortized Cost	$ 350,488
Due one year through five years, Amortized Cost	353,814
Due after ten years, Amortized Cost	11,400
Total, Amortized Cost	715,702
Due in one year or less, Estimated Fair Value	350,605
Due one year through five years, Estimated Fair Value	352,447
Due after ten years, Estimated Fair value	10,898
Total, Estimated fair value	$ 713,950