Share-Based Payments (Details 3) (Stock Option Stock Appreciation Rights, USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Stock Option Stock Appreciation Rights
Number of Options and SAR
Outstanding at the beginning of the period (in shares)	2,080,749
Granted (in shares)	866,300
Exercised (in shares)	(68,896)
Forfeited or expired (in shares)	(11,656)
Outstanding at the end of the period (in shares)	2,866,497	2,080,749
Vested and expected to vest (in shares)	2,793,934	2,041,026
Exercisable (in shares)	913,327	626,548
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 7.93
Granted (in dollars per share)	$ 9.22
Exercised (in dollars per share)	$ 2.13
Forfeited or expired (in dollars per share)	$ 7.95
Outstanding at the end of the period (in dollars per share)	$ 8.46	$ 7.93
Vested and expected to vest (in dollars per share)	$ 8.45	$ 7.91
Exercisable (in dollars per share)	$ 7.54	$ 6.40
Weighted-Average Remaining Contractual Term
Outstanding at the end of the period	8 years 6 months 4 days	8 years 15 days
Vested and expected to vest	8 years 5 months 27 days	8 years 11 days
Exercisable	7 years 5 months 1 day	6 years 10 months 28 days