Accounting for Shared-Based Payments (Details) (USD $)	3 Months Ended		88 Months Ended			3 Months Ended		88 Months Ended	3 Months Ended		88 Months Ended	3 Months Ended		88 Months Ended	3 Months Ended		88 Months Ended	3 Months Ended		88 Months Ended	3 Months Ended		88 Months Ended	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013	Jan. 17, 2013 2008 Equity Compensation Incentive Plan	Dec. 31, 2012 2008 Equity Compensation Incentive Plan	Mar. 31, 2013 Employees	Mar. 31, 2012 Employees	Mar. 31, 2013 Employees	Mar. 31, 2013 Employees Research and development	Mar. 31, 2012 Employees Research and development	Mar. 31, 2013 Employees Research and development	Mar. 31, 2013 Employees General and administrative	Mar. 31, 2012 Employees General and administrative	Mar. 31, 2013 Employees General and administrative	Mar. 31, 2013 Non-employees	Mar. 31, 2012 Non-employees	Mar. 31, 2013 Non-employees	Mar. 31, 2013 Non-employees Research and development	Mar. 31, 2012 Non-employees Research and development	Mar. 31, 2013 Non-employees Research and development	Mar. 31, 2013 Non-employees General and administrative	Mar. 31, 2012 Non-employees General and administrative	Mar. 31, 2013 Non-employees General and administrative	Mar. 31, 2013 Stock options	Mar. 31, 2012 Stock options
Shared-based payments
Amount of excess tax benefits recognized	$ 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)	1,257,000	438,000	6,741,000			726,000	210,000	3,736,000	276,000	116,000	1,877,000	450,000	94,000	1,859,000	531,000	228,000	3,005,000	135,000	0	432,000	396,000	228,000	2,573,000
Unrecognized compensation cost related to non-vested employee stock options (in dollars)																								$ 8,300,000
Weighted-average remaining vesting period over which unrecognized compensation cost is to be recognized																								2 years 4 months 24 days
Number of stock options which vest upon a change of control																								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)																								0.04%	1.05%
Risk-free interest rate, maximum (as a percent)																								1.87%	1.50%
Expected volatility (as a percent)																								60.00%	60.00%
Expected term																								6 years	6 years