Fair Value Measurements Fair Value, Assets and Liabilities Measured on a Recurring Basis (details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Value
Marketable Securities [Abstract]
Total	$ 59,309	$ 158,672
Fair Value, Inputs, Level 1
Marketable Securities [Abstract]
Total	48,819	145,543
Significant Other Observable Inputs (Level 2)
Marketable Securities [Abstract]
Total	10,490	13,129
Marketable Securities | Fair Value
Marketable Securities [Abstract]
U.S. treasury securities	35,005	40,015
Certificates of deposit	10,490	11,738
Mutual funds	466	387
Total marketable securities	45,961	52,140
Marketable Securities | Fair Value, Inputs, Level 1
Marketable Securities [Abstract]
U.S. treasury securities	35,005	40,015
Certificates of deposit	0	0
Mutual funds	466	387
Total marketable securities	35,471	40,402
Marketable Securities | Significant Other Observable Inputs (Level 2)
Marketable Securities [Abstract]
U.S. treasury securities	0	0
Certificates of deposit	10,490	11,738
Mutual funds	0	0
Total marketable securities	10,490	11,738
Cash Equivalents | Fair Value
Cash Equivalents [Abstract]
Money market funds	8,144	105,141
US Treasury Securities (Cash Equivalent), Fair Value Disclosure	5,204
Certificates of Deposit (Cash Equivalents), Fair Value Disclosure		1,391
Cash Equivalents, Fair Value Disclosure	13,348	106,532
Cash Equivalents | Fair Value, Inputs, Level 1
Cash Equivalents [Abstract]
Money market funds	8,144	105,141
US Treasury Securities (Cash Equivalent), Fair Value Disclosure	5,204
Certificates of Deposit (Cash Equivalents), Fair Value Disclosure		0
Cash Equivalents, Fair Value Disclosure	13,348	105,141
Cash Equivalents | Significant Other Observable Inputs (Level 2)
Cash Equivalents [Abstract]
Money market funds	0	0
US Treasury Securities (Cash Equivalent), Fair Value Disclosure	0
Certificates of Deposit (Cash Equivalents), Fair Value Disclosure		1,391
Cash Equivalents, Fair Value Disclosure	$ 0	$ 1,391