Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Description
Available-for-sale securities	$ 97,938	$ 31,528
Transfers between Level 1 and Level 2 of the fair value hierarchy	0		0
Recurring basis | Fair Value
Description
Total assets fair value	237,463	145,396
Recurring basis | Fair Value | Money market funds
Description
Cash and cash equivalents	139,525	111,368
Recurring basis | Fair Value | U.S. Treasury securities
Description
Available-for-sale securities	503	15,052
Recurring basis | Fair Value | U.S. government-sponsored securities
Description
Cash and cash equivalents		2,500
Available-for-sale securities	97,435	16,476
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Description
Total assets fair value	140,028	126,420
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Description
Cash and cash equivalents	139,525	111,368
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury securities
Description
Available-for-sale securities	503	15,052
Recurring basis | Significant Other Observable Inputs (Level 2)
Description
Total assets fair value	97,435	18,976
Recurring basis | Significant Other Observable Inputs (Level 2) | U.S. government-sponsored securities
Description
Cash and cash equivalents		2,500
Available-for-sale securities	$ 97,435	$ 16,476