Accounting for Shared-Based Payments (Details) (USD $)	3 Months Ended		100 Months Ended			3 Months Ended		100 Months Ended	3 Months Ended		100 Months Ended	3 Months Ended		100 Months Ended	3 Months Ended		100 Months Ended	3 Months Ended		100 Months Ended	3 Months Ended		100 Months Ended	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Jan. 17, 2013 2008 Equity Compensation Incentive Plan	Dec. 31, 2012 2008 Equity Compensation Incentive Plan	Mar. 31, 2014 Employees	Mar. 31, 2013 Employees	Mar. 31, 2014 Employees	Mar. 31, 2014 Employees Research and development	Mar. 31, 2013 Employees Research and development	Mar. 31, 2014 Employees Research and development	Mar. 31, 2014 Employees General and administrative	Mar. 31, 2013 Employees General and administrative	Mar. 31, 2014 Employees General and administrative	Mar. 31, 2014 Non-employees	Mar. 31, 2013 Non-employees	Mar. 31, 2014 Non-employees	Mar. 31, 2014 Non-employees Research and development	Mar. 31, 2013 Non-employees Research and development	Mar. 31, 2014 Non-employees Research and development	Mar. 31, 2014 Non-employees General and administrative	Mar. 31, 2013 Non-employees General and administrative	Mar. 31, 2014 Non-employees General and administrative	Mar. 31, 2014 Stock options	Mar. 31, 2013 Stock options	Mar. 31, 2014 Stock options Minimum	Mar. 31, 2014 Stock options Maximum
Shared-based payments
Amount of excess tax benefits recognized (in dollars)	$ 0
Vesting period																								3 years
Contractual term																								10 years
Number of shares reserved under the plan				15,000,000	7,500,000
Total stock-based compensation expense (in dollars)	1,264,000	1,257,000	11,362,000			822,000	726,000	6,683,000	426,000	276,000	3,346,000	396,000	450,000	3,337,000	442,000	531,000	4,679,000	6,000	135,000	448,000	436,000	396,000	4,231,000
Unrecognized compensation cost related to non-vested employee stock options, net of expected forfeitures (in dollars)																								$ 8,000,000
Weighted-average remaining vesting period over which unrecognized compensation cost is to be recognized																								1 year 6 months
Number of stock options which vest upon a change of control (in shares)																								4,364,000
Weighted-average assumptions used to estimate fair value of stock option awards using the Black-Scholes option valuation model
Risk-free interest rate, minimum (as a percent)																								1.94%	0.04%
Risk-free interest rate, maximum (as a percent)																								2.52%	1.87%
Expected volatility (as a percent)																								60.00%	60.00%
Expected term																									6 years	6 years	9 years 3 months 18 days