Accounting for Shared-Based Payments (Details) (USD $)	3 Months Ended		9 Months Ended		94 Months Ended			3 Months Ended		9 Months Ended		94 Months Ended	3 Months Ended		9 Months Ended		94 Months Ended	3 Months Ended		9 Months Ended		94 Months Ended	3 Months Ended		9 Months Ended		94 Months Ended	3 Months Ended		9 Months Ended		94 Months Ended	3 Months Ended		9 Months Ended		94 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Jan. 17, 2013 2008 Equity Compensation Incentive Plan	Dec. 31, 2012 2008 Equity Compensation Incentive Plan	Sep. 30, 2013 Employees	Sep. 30, 2012 Employees	Sep. 30, 2013 Employees	Sep. 30, 2012 Employees	Sep. 30, 2013 Employees	Sep. 30, 2013 Employees Research and development	Sep. 30, 2012 Employees Research and development	Sep. 30, 2013 Employees Research and development	Sep. 30, 2012 Employees Research and development	Sep. 30, 2013 Employees Research and development	Sep. 30, 2013 Employees General and administrative	Sep. 30, 2012 Employees General and administrative	Sep. 30, 2013 Employees General and administrative	Sep. 30, 2012 Employees General and administrative	Sep. 30, 2013 Employees General and administrative	Sep. 30, 2013 Non-employees	Sep. 30, 2012 Non-employees	Sep. 30, 2013 Non-employees	Sep. 30, 2012 Non-employees	Sep. 30, 2013 Non-employees	Sep. 30, 2013 Non-employees Research and development	Sep. 30, 2012 Non-employees Research and development	Sep. 30, 2013 Non-employees Research and development	Sep. 30, 2012 Non-employees Research and development	Sep. 30, 2013 Non-employees Research and development	Sep. 30, 2013 Non-employees General and administrative	Sep. 30, 2012 Non-employees General and administrative	Sep. 30, 2013 Non-employees General and administrative	Sep. 30, 2012 Non-employees General and administrative	Sep. 30, 2013 Non-employees General and administrative	Sep. 30, 2013 Stock options	Sep. 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)	1,090,000	553,000	3,258,000	1,345,000	8,742,000			774,000	380,000	2,108,000	835,000	5,120,000	388,000	226,000	942,000	488,000	2,544,000	386,000	154,000	1,166,000	347,000	2,576,000	316,000	173,000	1,150,000	510,000	3,622,000	3,000	3,000	140,000	3,000	436,000	313,000	170,000	1,010,000	507,000	3,186,000
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																																						1 year 10 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)																																						0.41%	0.92%
Risk-free interest rate, maximum (as a percent)																																						2.64%	1.50%
Expected volatility (as a percent)																																						60.00%	60.00%
Expected term																																						6 years	6 years