Subsequent Events (Details) (USD $)	1 Months Ended	12 Months Ended
	Jan. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Subsequent events
Net proceeds from the issuance of common stock	$ 124,100,000	$ 51,733,000	$ 32,414,000	$ 4,039,000
Capitalized expenses incurred in public offering		100,000
Subsequent event
Subsequent events
Common stock issue price (in dollars per share)	$ 50.00
Number of shares represents partial exercise of overallotment option granted to underwriters	345,000
Net proceeds from the issuance of common stock	$ 124,100,000
Subsequent event | Stock options
Subsequent events
Common stock issued (in shares)	2,645,000