Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013 Recurring basis Estimated fair value	Dec. 31, 2012 Recurring basis Estimated fair value	Jun. 30, 2013 Recurring basis Estimated fair value Money Market Funds	Dec. 31, 2012 Recurring basis Estimated fair value Money Market Funds	Jun. 30, 2013 Recurring basis Estimated fair value US government debt securities	Dec. 31, 2012 Recurring basis Estimated fair value US government debt securities	Jun. 30, 2013 Recurring basis Estimated fair value Corporate debt securities	Dec. 31, 2012 Recurring basis Estimated fair value Corporate debt securities	Jun. 30, 2013 Recurring basis Estimated fair value Foreign government debt securities	Dec. 31, 2012 Recurring basis Estimated fair value Foreign government debt securities	Jun. 30, 2013 Recurring basis Quoted Prices in Active Markets for Identical Assets (Level 1) Money Market Funds	Dec. 31, 2012 Recurring basis Quoted Prices in Active Markets for Identical Assets (Level 1) Money Market Funds	Dec. 31, 2012 Recurring basis Quoted Prices in Active Markets for Identical Assets (Level 1) US government debt securities	Jun. 30, 2013 Recurring basis Significant Other Observable Inputs (Level 2)	Dec. 31, 2012 Recurring basis Significant Other Observable Inputs (Level 2)	Jun. 30, 2013 Recurring basis Significant Other Observable Inputs (Level 2) US government debt securities	Dec. 31, 2012 Recurring basis Significant Other Observable Inputs (Level 2) US government debt securities	Jun. 30, 2013 Recurring basis Significant Other Observable Inputs (Level 2) Corporate debt securities	Dec. 31, 2012 Recurring basis Significant Other Observable Inputs (Level 2) Corporate debt securities	Jun. 30, 2013 Recurring basis Significant Other Observable Inputs (Level 2) Foreign government debt securities	Dec. 31, 2012 Recurring basis Significant Other Observable Inputs (Level 2) Foreign government debt securities
Fair Value Measurements
Maturity period of cash equivalents to classify as money market funds, maximum	3 months
Financial Assets:
Cash and cash equivalents				$ 287,465	$ 47,086							$ 287,465	$ 47,086
Investment, at fair value						35,000	59,815	38,839	110,508					55,817			35,000	3,998	38,839	110,508
Cash and cash equivalents, and short-term investments										3,603	2,421										3,603	2,421
Financial Liabilities:
Debt		225,000	95,000												225,000	95,000
Transfers of assets from level 1 to level 2 of fair value hierarchy	0
Transfers of assets from level 2 to level 1 of fair value hierarchy	$ 0