Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified																															3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Money market funds	Dec. 31, 2013 Money market funds	Mar. 31, 2014 Corporate notes, bonds and commercial paper	Dec. 31, 2013 Corporate notes, bonds and commercial paper	Mar. 31, 2014 Recurring basis Total	Dec. 31, 2013 Recurring basis Total	Mar. 31, 2014 Recurring basis Total Money market funds	Dec. 31, 2013 Recurring basis Total Money market funds	Mar. 31, 2014 Recurring basis Total Corporate notes, bonds and commercial paper	Dec. 31, 2013 Recurring basis Total Corporate notes, bonds and commercial paper	Mar. 31, 2014 Recurring basis Quoted Market Prices in Active Markets (Level 1)	Dec. 31, 2013 Recurring basis Quoted Market Prices in Active Markets (Level 1)	Mar. 31, 2014 Recurring basis Quoted Market Prices in Active Markets (Level 1) Money market funds	Dec. 31, 2013 Recurring basis Quoted Market Prices in Active Markets (Level 1) Money market funds	Mar. 31, 2014 Recurring basis Quoted Market Prices in Active Markets (Level 1) Corporate notes, bonds and commercial paper	Dec. 31, 2013 Recurring basis Quoted Market Prices in Active Markets (Level 1) Corporate notes, bonds and commercial paper	Mar. 31, 2014 Recurring basis Significant Other Observable Inputs (Level 2)	Dec. 31, 2013 Recurring basis Significant Other Observable Inputs (Level 2)	Mar. 31, 2014 Recurring basis Significant Other Observable Inputs (Level 2) Money market funds	Dec. 31, 2013 Recurring basis Significant Other Observable Inputs (Level 2) Money market funds	Mar. 31, 2014 Recurring basis Significant Other Observable Inputs (Level 2) Corporate notes, bonds and commercial paper	Dec. 31, 2013 Recurring basis Significant Other Observable Inputs (Level 2) Corporate notes, bonds and commercial paper	Mar. 31, 2014 Recurring basis Significant Unobservable Inputs (Level 3)	Dec. 31, 2013 Recurring basis Significant Unobservable Inputs (Level 3)	Mar. 31, 2014 Recurring basis Significant Unobservable Inputs (Level 3) Money market funds	Dec. 31, 2013 Recurring basis Significant Unobservable Inputs (Level 3) Money market funds	Mar. 31, 2014 Recurring basis Significant Unobservable Inputs (Level 3) Corporate notes, bonds and commercial paper	Dec. 31, 2013 Recurring basis Significant Unobservable Inputs (Level 3) Corporate notes, bonds and commercial paper	Mar. 31, 2014 Nonrecurring basis	Mar. 31, 2014 Nonrecurring basis Quoted Market Prices in Active Markets (Level 1)	Mar. 31, 2014 Nonrecurring basis Significant Other Observable Inputs (Level 2)	Mar. 31, 2014 Nonrecurring basis Significant Unobservable Inputs (Level 3)
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	$ 376,116	$ 359,097	$ 317,769	$ 300,605	$ 58,347	$ 58,492	$ 376,116	$ 359,097	$ 317,769	$ 300,605	$ 58,347	$ 58,492	$ 317,769	$ 300,605	$ 317,769	$ 300,605	$ 0	$ 0	$ 58,347	$ 58,492	$ 0	$ 0	$ 58,347	$ 58,492	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Investment in non-marketable equity securities
Carrying Value																															600
Fair Value																																0	0	600
Impairment of investment in non-marketable equity security																															$ 0