Organization (Details) - Range [Domain] - USD ($) $ / shares in Units, $ in Thousands						6 Months Ended
	Jul. 28, 2015	Jan. 28, 2015	Oct. 07, 2014	Jun. 25, 2014	Jun. 11, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Organization
Accumulated deficit						$ 384,260		$ 344,238
Cash, cash equivalents and marketable securities						224,000
Initial public offering
Proceeds from the sale of common stock						$ 158,414	$ 51,332
Common Stock
Initial public offering
Shares sold		4,000,000	2,750,000		6,500,000
Share issue price (in dollars per share)		$ 36.75	$ 18.25		$ 8.00
Proceeds from the sale of common stock		$ 137,800	$ 46,900
Options exercised by underwriters (in shares)			378,524	512,744
Aggregate proceeds received, net of underwriting discounts and commissions and offering costs		$ 158,400	$ 53,400	$ 50,400
Common Stock | Underwriters
Initial public offering
Options exercised by underwriters (in shares)		600,000
Common Stock | Subsequent Event
Initial public offering
Shares sold	4,054,054
Share issue price (in dollars per share)	$ 74.00
Proceeds from the sale of common stock	$ 281,500
Aggregate proceeds received, net of underwriting discounts and commissions and offering costs	$ 323,800
Common Stock | Subsequent Event | Underwriters
Initial public offering
Options exercised by underwriters (in shares)	608,108