Income Taxes (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Percent of pre-tax income:
U.S. federal statutory income tax rate (as a percent)	35.00%	35.00%	35.00%
State taxes, net of federal benefit (as a percent)			3.40%
Permanent items (as a percent)	(2.20%)	(1.00%)	(0.60%)
Research and development credit (as a percent)	2.70%	1.00%	0.70%
Change in valuation allowance (as a percent)	(35.50%)	(35.00%)	(33.30%)
Effective income tax rate (as a percent)			5.20%
Deferred tax assets (liabilities)
Federal net operating loss carryforwards	$ 30,084	$ 29,464
State and local net operating loss carryforwards	5,026	4,825
License and technology payments	6,407	5,566
Share-based compensation	1,396	476
Depreciation	(7)	(11)
Federal research and development credit carryforwards	2,966	1,562
State research and development credit carryforwards	1,483	781
Deferred income tax assets	47,355	42,663
Valuation allowance	$ (47,355)	$ (42,663)