Accounting for Shared-Based Payments (Details) (USD $)	12 Months Ended			86 Months Ended			12 Months Ended			86 Months Ended	12 Months Ended			86 Months Ended	12 Months Ended			86 Months Ended	12 Months Ended			86 Months Ended	12 Months Ended			86 Months Ended	12 Months Ended			86 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012	Jan. 17, 2013 2008 Equity Compensation Incentive Plan	Dec. 31, 2012 2008 Equity Compensation Incentive Plan	Dec. 31, 2012 Employees	Dec. 31, 2011 Employees	Dec. 31, 2010 Employees	Dec. 31, 2012 Employees	Dec. 31, 2012 Employees Research and development	Dec. 31, 2011 Employees Research and development	Dec. 31, 2010 Employees Research and development	Dec. 31, 2012 Employees Research and development	Dec. 31, 2012 Employees General and administrative	Dec. 31, 2011 Employees General and administrative	Dec. 31, 2010 Employees General and administrative	Dec. 31, 2012 Employees General and administrative	Dec. 31, 2012 Non-employees	Dec. 31, 2011 Non-employees	Dec. 31, 2010 Non-employees	Dec. 31, 2012 Non-employees	Dec. 31, 2012 Non-employees Research and development	Dec. 31, 2011 Non-employees Research and development	Dec. 31, 2010 Non-employees Research and development	Dec. 31, 2012 Non-employees Research and development	Dec. 31, 2012 Non-employees General and administrative	Dec. 31, 2011 Non-employees General and administrative	Dec. 31, 2010 Non-employees General and administrative	Dec. 31, 2012 Non-employees General and administrative	Dec. 31, 2012 Stock options	Dec. 31, 2011 Stock options	Dec. 31, 2010 Stock options
Shared-based payments
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)	$ 2,515,000	$ 822,000	$ 712,000	$ 5,484,000			$ 1,610,000	$ 200,000	$ 398,000	$ 3,011,000	$ 975,000	$ 107,000	$ 188,000	$ 1,602,000	$ 635,000	$ 93,000	$ 210,000	$ 1,409,000	$ 905,000	$ 622,000	$ 314,000	$ 2,473,000	$ 128,000	$ 73,000	$ 52,000	$ 296,000	$ 777,000	$ 549,000	$ 262,000	$ 2,177,000
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)																															0.85%	0.88%	2.31%
Risk-free interest rate, maximum (as a percent)																															1.50%	1.25%	2.71%
Expected volatility (as a percent)																															60.00%	70.00%	90.00%
Expected term																															6 years	6 years	6 years
Weighted-average fair value of all options granted (in dollars per share)																															$ 2.38	$ 2.09	$ 6.77
Unrecognized compensation cost related to non-vested employee stock options (in dollars)																															$ 9,201,000
Weighted-average remaining vesting period over which unrecognized compensation cost is to be recognized																															3 years