Stock-based compensation (Details 2) (USD $)	3 Months Ended	12 Months Ended	24 Months Ended	3 Months Ended						24 Months Ended	3 Months Ended		24 Months Ended	3 Months Ended	0 Months Ended	3 Months Ended	24 Months Ended	3 Months Ended	0 Months Ended
	Mar. 31, 2013 Stock options	Dec. 31, 2012 Stock options	Mar. 31, 2013 Stock options	Mar. 31, 2013 Stock options Minimum	Mar. 31, 2012 Stock options Minimum	Mar. 31, 2013 Stock options Maximum	Mar. 31, 2012 Stock options Maximum	Mar. 31, 2013 Stock options Employees	Mar. 31, 2012 Stock options Employees	Mar. 31, 2013 Stock options Employees	Mar. 31, 2013 Stock options Non-employees	Mar. 31, 2012 Stock options Non-employees	Mar. 31, 2013 Stock options Non-employees	Mar. 31, 2013 Restricted stock units	Dec. 05, 2012 Restricted stock units CEO	Mar. 31, 2013 Restricted stock units CEO	Mar. 31, 2013 Restricted stock units CEO	Mar. 31, 2013 Time based restricted stock units	Dec. 05, 2012 Time based restricted stock units CEO	Dec. 05, 2012 Performance based restricted stock units CEO
Shares
Outstanding at the beginning of the period (in shares)	1,218,153
Granted (in shares)	437,542		1,774,111					437,542	60,799		12,356		205,024
Exercised (in shares)	(24,049)
Outstanding at the end of the period (in shares)	1,631,646	1,218,153	1,631,646
Exercisable at the end of the period (in shares)	227,503		227,503
Vested and expected to vest at the end of the period (in shares)	1,487,084		1,487,084
Weighted average exercise price per share
Outstanding at the beginning of the period (in dollars per share)	$ 3.83
Granted (in dollars per share)	$ 12.78							$ 12.78	$ 4.01			$ 4.01	$ 0.96
Exercised (in dollars per share)	$ 0.04
Outstanding at the end of period (in dollars per share)	$ 6.28	$ 3.83	$ 6.28
Exercisable at the end of the period (in dollars per share)	$ 1.55		$ 1.55
Vested and expected to vest at the end of the period (in dollars per share)	$ 6.22		$ 6.22
Weighted average remaining contractual term
Outstanding at the end of the period	9 years 3 months 22 days	9 years 3 months 25 days
Exercisable at the end of the period	8 years 9 months
Vested and expected to vest at the end of the period	8 years 9 months
Outstanding at the end of the period	9 years 3 months 22 days	9 years 3 months 25 days
Aggregate intrinsic value
Outstanding at the end of period	$ 5,535,000
Exercisable at the end of period	1,694,000		1,694,000
Vested and expected to vest at the end of the period	5,652,000		5,652,000
Outstanding at the end of period	6,091,000	5,535,000	6,091,000
Shares
Unvested at the beginning of the period (in shares)														192,308
Granted (in shares)															192,308				128,205	64,103
Vested (in shares)														16,025		16,025
Unvested at the end of the period (in shares)														176,283
Weighted-Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share)														$ 8.80
Granted (in dollars per share)																			$ 7.80
Vested (in dollars per share)														$ 7.80
Unvested at the end of the period (in dollars per share)														$ 8.80
Assumptions used to estimate fair value of each stock-based award on the grant date using the Black-Scholes option pricing model
Risk free interest rate (as a percent)				0.90%	1.20%	1.10%	1.84%
Volatility (as a percent)				89.00%	79.00%	91.00%	82.00%
Expected term				5 years 3 months 18 days	6 years	9 years 8 months 12 days	9 years 9 months
Additional disclosures
Weighted average grant-date fair value of options granted (in dollars per share)								$ 9.42	$ 2.84
Weighted average exercise price (in dollars per share)																$ 0.001
Stock-based compensation expense								342,000	44,000	688,000	363,000	215,000	1,362,000			132,000	168,000
Total unrecognized compensation cost related to non-vested stock options and restricted stock (in dollars)	$ 9,434,000		$ 9,434,000															$ 1,525,000
Weighted average period for recognition of compensation cost related to unvested stock awards	3 years 2 months 12 days																	1 year 9 months 18 days
Vested (in shares)														16,025		16,025
Future quarterly restricted stock unit award grant																			16,025