Share-Based Payments (Details 3) (Stock option, USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Stock option
Number of Options
Outstanding at the beginning of the period (in shares)	569,911
Granted (in shares)	899,332
Exercised (in shares)	(33,365)
Forfeited or expired (in shares)	(7,718)
Outstanding at the end of the period (in shares)	1,428,160	569,911
Vested and expected to vest (in shares)	1,377,755	564,083
Exercisable (in shares)	170,339	200,312
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 5.72
Granted (in dollars per share)	$ 7.89
Exercised (in dollars per share)	$ 0.62
Forfeited or expired (in dollars per share)	$ 6.88
Outstanding at the end of the period (in dollars per share)	$ 7.20	$ 5.72
Vested and expected to vest (in dollars per share)	$ 7.18	$ 5.72
Exercisable (in dollars per share)	$ 2.44	$ 2.11
Weighted-Average Remaining Contractual Term
Outstanding at the end of the period	9 years	7 years 10 months 17 days
Vested and expected to vest	8 years 11 months 26 days	7 years 10 months 13 days
Exercisable	5 years 11 months 23 days	5 years 8 months 23 days