Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities		$ 226,381		$ 359,097
Investment in non-marketable equity securities
Impairment of investment in non-marketable equity security	1,400	0	1,400
Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities		79,522		300,605
Corporate notes, bonds and commercial paper
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities		146,859		58,492
Recurring basis | Total
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities		226,381		359,097
Recurring basis | Total | Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities		79,522		300,605
Recurring basis | Total | Corporate notes, bonds and commercial paper
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities		146,859		58,492
Recurring basis | Quoted Market Prices in Active Markets (Level 1)
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities		79,522		300,605
Recurring basis | Quoted Market Prices in Active Markets (Level 1) | Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities		79,522		300,605
Recurring basis | Quoted Market Prices in Active Markets (Level 1) | Corporate notes, bonds and commercial paper
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities		0		0
Recurring basis | Significant Other Observable Inputs (Level 2)
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities		146,859		58,492
Recurring basis | Significant Other Observable Inputs (Level 2) | Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities		0		0
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate notes, bonds and commercial paper
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities		146,859		58,492
Recurring basis | Significant Unobservable Inputs (Level 3)
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities		0		0
Recurring basis | Significant Unobservable Inputs (Level 3) | Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities		0		0
Recurring basis | Significant Unobservable Inputs (Level 3) | Corporate notes, bonds and commercial paper
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities		0		0
Nonrecurring basis
Investment in non-marketable equity securities
Carrying Value		600
Impairment of investment in non-marketable equity security		0
Nonrecurring basis | Quoted Market Prices in Active Markets (Level 1)
Investment in non-marketable equity securities
Fair Value		0
Nonrecurring basis | Significant Other Observable Inputs (Level 2)
Investment in non-marketable equity securities
Fair Value		0
Nonrecurring basis | Significant Unobservable Inputs (Level 3)
Investment in non-marketable equity securities
Fair Value		$ 600