Significant Accounting Policies (Details 7) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Allocation of stock-based compensation expense
Stock-based compensation expense	$ 2,869,000	$ 2,267,000
Non-vested stock option
Allocation of stock-based compensation expense
Unrecognized estimated non-cash stock-based compensation expense	10,700,000
Weighted average period over which cost is expected to be recognized	1 year 4 months 24 days
RSUs
Allocation of stock-based compensation expense
Unrecognized estimated non-cash stock-based compensation expense	3,800,000
Weighted average period over which cost is expected to be recognized	2 years 1 month 6 days
Research and development
Allocation of stock-based compensation expense
Stock-based compensation expense	2,546,000	1,935,000
General and administrative
Allocation of stock-based compensation expense
Stock-based compensation expense	$ 323,000	$ 332,000