Accounting for Shared-Based Payments (Details) (USD $)	3 Months Ended		6 Months Ended		91 Months Ended			3 Months Ended		6 Months Ended		91 Months Ended	3 Months Ended		6 Months Ended		91 Months Ended	3 Months Ended		6 Months Ended		91 Months Ended	3 Months Ended		6 Months Ended		91 Months Ended	3 Months Ended	6 Months Ended	91 Months Ended	3 Months Ended		6 Months Ended		91 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jan. 17, 2013 2008 Equity Compensation Incentive Plan	Dec. 31, 2012 2008 Equity Compensation Incentive Plan	Jun. 30, 2013 Employees	Jun. 30, 2012 Employees	Jun. 30, 2013 Employees	Jun. 30, 2012 Employees	Jun. 30, 2013 Employees	Jun. 30, 2013 Employees Research and development	Jun. 30, 2012 Employees Research and development	Jun. 30, 2013 Employees Research and development	Jun. 30, 2012 Employees Research and development	Jun. 30, 2013 Employees Research and development	Jun. 30, 2013 Employees General and administrative	Jun. 30, 2012 Employees General and administrative	Jun. 30, 2013 Employees General and administrative	Jun. 30, 2012 Employees General and administrative	Jun. 30, 2013 Employees General and administrative	Jun. 30, 2013 Non-employees	Jun. 30, 2012 Non-employees	Jun. 30, 2013 Non-employees	Jun. 30, 2012 Non-employees	Jun. 30, 2013 Non-employees	Jun. 30, 2013 Non-employees Research and development	Jun. 30, 2013 Non-employees Research and development	Jun. 30, 2013 Non-employees Research and development	Jun. 30, 2013 Non-employees General and administrative	Jun. 30, 2012 Non-employees General and administrative	Jun. 30, 2013 Non-employees General and administrative	Jun. 30, 2012 Non-employees General and administrative	Jun. 30, 2013 Non-employees General and administrative	Jun. 30, 2013 Stock options	Jun. 30, 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)	911,000	354,000	2,168,000	792,000	7,652,000			609,000	246,000	1,336,000	456,000	4,346,000	278,000	146,000	555,000	262,000	2,156,000	331,000	100,000	781,000	194,000	2,190,000	302,000	108,000	832,000	337,000	3,306,000	2,000	137,000	433,000	300,000	108,000	695,000	336,000	2,873,000
Unrecognized compensation cost related to non-vested employee stock options (in dollars)																																				$ 7,400,000
Weighted-average remaining vesting period over which unrecognized compensation cost is to be recognized																																				2 years
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)																																				0.41%	0.97%
Risk-free interest rate, maximum (as a percent)																																				1.11%	1.50%
Expected volatility (as a percent)																																				60.00%	60.00%
Expected term																																				6 years	6 years