Common Stock (Details) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) Vote shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)	Mar. 31, 2017 USD ($)
Common Stock, Capital Shares Reserved for Future Issuance		7,621,134	6,309,915
Proceeds from public offering of common stock, net of issuance costs | $		$ 154,143	$ 97,488
March 2017 ATM program | Cowen and Company LLC (“Cowen)
Aggregate sales proceeds of stock | $					$ 50,000
Commission payable (as a percent)					3.00%
Common Stock.
Voting rights per share | Vote		1
Dividends, Common Stock | $		$ 0	$ 0	$ 0
Number of common stock issued			6,785,000
Common Stock. | Outstanding stock options
Common Stock, Capital Shares Reserved for Future Issuance		225,000
2013 Plan | Common Stock. | Outstanding stock options
Common Stock, Capital Shares Reserved for Future Issuance		1,220,567	1,595,082
2015 Plan | Common Stock.
Common Stock, Capital Shares Reserved for Future Issuance		2,502,338	2,760,472
2015 Plan | Common Stock. | Outstanding stock options
Common Stock, Capital Shares Reserved for Future Issuance		2,921,987	1,569,746
2015 Employee Stock Purchase Plan
Common Stock, Capital Shares Reserved for Future Issuance		751,242	384,615
Subsequent Event
Price per share | $ / shares	$ 29.76
Subsequent Event | March 2017 ATM program | Cowen and Company LLC (“Cowen)
Number of common stock issued	1,429,205
Price per share | $ / shares	$ 34.99
Proceeds from public offering of common stock, net of issuance costs | $	$ 50,000