Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Assets:
Marketable securities	$ 50,983	$ 48,206
Liabilities:
Warrant liability	172	251
Quoted Prices in Active Markets (Level 1)
Assets:
Cash and cash equivalents	15,903	31,561
Total assets at fair value	15,903	31,561
Significant Other Observable Inputs (Level 2)
Assets:
Cash and cash equivalents	3,006	8,741
Marketable securities	50,983	48,206
Marketable securities - restricted (Other Assets)	308	279
Total assets at fair value	54,297	57,226
Significant Unobservable Inputs (Level 3)
Liabilities:
Warrant liability	172	251
Total Carrying Value
Assets:
Cash and cash equivalents	18,909	40,302
Marketable securities	50,983	48,206
Marketable securities - restricted (Other Assets)	308	279
Total assets at fair value	70,200	88,787
Liabilities:
Warrant liability	$ 172	$ 251