Stock-based compensation (Details) (USD $)	3 Months Ended	0 Months Ended			1 Months Ended		3 Months Ended		12 Months Ended	3 Months Ended								12 Months Ended	0 Months Ended	3 Months Ended					0 Months Ended
	Mar. 31, 2014	Dec. 05, 2012 Time based restricted stock units CEO	Mar. 20, 2013 Performance based restricted stock units CEO	Dec. 05, 2012 Performance based restricted stock units CEO	Dec. 31, 2013 Performance based restricted stock units CEO	Jan. 31, 2014 Founder item	Mar. 31, 2014 Founder	Mar. 31, 2014 Stock options	Dec. 31, 2013 Stock options	Mar. 31, 2014 Stock options Minimum	Mar. 31, 2013 Stock options Minimum	Mar. 31, 2014 Stock options Maximum	Mar. 31, 2013 Stock options Maximum	Mar. 31, 2013 Stock options Employees	Mar. 31, 2014 Stock options Senior Executive	Mar. 31, 2014 Restricted stock units	Mar. 31, 2013 Restricted stock units	Dec. 31, 2013 Restricted stock units	Dec. 05, 2012 Restricted stock units CEO	Mar. 31, 2014 Restricted stock units CEO	Dec. 31, 2013 Restricted stock units CEO	Sep. 30, 2013 Restricted stock units CEO	Jun. 30, 2013 Restricted stock units CEO	Mar. 31, 2013 Restricted stock units CEO	Feb. 07, 2014 Restricted stock units Performance based restricted stock units
Shares
Unvested at the beginning of the period (in shares)						987,081	987,081
Granted (in shares)		128,205		64,103	32,051			377,098											192,308
Vested (in shares)					(19,230)		(164,515)													(16,025)	(16,025)	(16,025)	(16,025)	(16,025)
Unvested at the end of the period (in shares)							822,566
Number of holders transmitted from employee to non-employee						1
Additional share-based compensation expense with respect to transmission of holder from employee to non-employee							$ 300,000
Shares
Outstanding at the beginning of the period (in shares)								2,413,237
Granted (in shares)								377,098						437,542
Exercised (in shares)								(109,866)
Forfeited (in shares)								(510,440)
Outstanding at the end of the period (in shares)								2,170,029	2,413,237
Exercisable at the end of the period (in shares)								537,544
Vested and expected to vest at the end of the period (in shares)								1,400,912
Weighted average exercise price per share
Outstanding at the beginning of the period (in dollars per share)								$ 9.26
Granted (in dollars per share)		$ 7.80	$ 10.00					$ 10.09						$ 12.78
Exercised (in dollars per share)								$ 0.04
Forfeited (in dollars per share)								$ 12.87
Outstanding at the end of period (in dollars per share)								$ 8.95	$ 9.26
Exercisable at the end of the period (in dollars per share)								$ 3.46
Vested and expected to vest at the end of the period (in dollars per share)								$ 8.37
Weighted average remaining contractual term
Outstanding at the beginning of the period								8 years 8 months 1 day	9 years 18 days
Outstanding at the end of the period								8 years 8 months 1 day	9 years 18 days
Exercisable at the end of the period								8 years 3 months 11 days							1 year
Vested and expected to vest at the end of the period								8 years 4 months 6 days
Aggregate intrinsic value
Outstanding at the beginning of the period								6,087,000
Outstanding at the end of period								3,753,000	6,087,000
Exercisable at the end of period								3,112,000
Vested and expected to vest at the end of the period								3,094,000
Total intrinsic value								1,100,000
Weighted average grant date fair value (in dollars per share)								$ 7.06						$ 9.42
Assumptions used to estimate fair value of each stock-based option award on the grant date using the Black-Scholes option pricing model
Risk free interest rate (as a percent)										1.60%	0.90%	1.80%	1.10%
Volatility (as a percent)										78.00%	89.00%	84.00%	91.00%
Expected term										5 years 3 months 18 days	5 years 3 months 18 days	5 years 9 months 18 days	9 years 8 months 12 days
Additional disclosures
Development stage product development revenues	0
Total unrecognized compensation cost, net of estimated forfeitures, related to unvested stock options and restricted stock units (in dollars)	8,600,000
Weighted average period for recognition of compensation cost related to unvested stock options and restricted stock units	2 years 5 months 12 days
Vesting period accelerated															12 days
Number of options vested for vesting period accelerated															92,223
Stock-based compensation expense, net reversed															600,000
Vested in period fair value																		800,000
Weighted average grant-date fair value of options granted (in dollars per share)																									$ 8.75
Share-based compensation expense																$ 200,000	$ 100,000