Stock-Based Compensation (Details 2) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Number of Shares
Options outstanding (in shares)	4,400,106
Granted (in shares)	619,150
Exercised (in shares)	(178,071)
Forfeited or expired (in shares)	(19,750)
Options outstanding (in shares)	4,821,435
Vested and expected to vest (in shares)	4,593,714
Exercisable (in shares)	1,552,681
Weighted Average Exercise Price
Options outstanding (in dollars per share)	$ 10.59
Granted (in dollars per share)	$ 16.83
Exercised (in dollars per share)	$ 8.28
Forfeited or expired (in dollars per share)	$ 14.49
Options outstanding (in dollars per share)	$ 11.46
Vested and expected to vest (in dollars per share)	$ 11.33
Exercisable (in dollars per share)	$ 8.25
Weighted Average Remaining Contractual Life
Options outstanding	8 years 2 months 9 days
Vested and expected to vest	8 years 2 months 1 day
Exercisable	7 years 8 months 19 days
Aggregate Intrinsic Value
Options outstanding (in dollars)	$ 45,030,000
Vested and expected to vest (in dollars)	43,494,000
Exercisable (in dollars)	19,496,000
Stock options
Fair value and assumptions used in determining fair value of stock options
Dividend yield (as a percent)	0.00%	0.00%
Expected option term	6 years 3 months	6 years 3 months
Weighted-average fair value of stock options granted (in dollars per share)	$ 11.66	$ 14.35
Stock options | Minimum
Fair value and assumptions used in determining fair value of stock options
Volatility (as a percent)	79.00%	84.00%
Risk-free interest rate (as a percent)	1.31%	1.46%
Stock options | Maximum
Fair value and assumptions used in determining fair value of stock options
Volatility (as a percent)	82.00%	86.00%
Risk-free interest rate (as a percent)	1.57%	1.76%
Performance shares
Stock Option disclosures
Non-cash compensation expense	$ 1,500,000
Number of Shares
Options outstanding (in shares)	193,334