Stockholders' Equity - Narrative (Details) $ / shares in Units, $ in Thousands			12 Months Ended
	May 25, 2017 USD ($) $ / shares shares	Sep. 15, 2016 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) vote shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Mar. 01, 2019 shares
Class of Stock [Line Items]
Proceeds from sale of common stock | $			$ 135,972	$ 134,215	$ 167,383
Number of voting rights | vote			1
Common Stock
Class of Stock [Line Items]
Proceeds from sale of common stock | $	$ 72,700		$ 136,400	$ 61,100
New shares issued under shelf registration (in shares)	1,400,000		1,300,000
Common stock issued and sold under sales agreement (in shares)				1,100,000	4,200,000
Price per share issued under underwriting agreement (in dollars per share) | $ / shares	$ 53.75
RBC Capital Markets LLC and Cantor Fitzgerald and Co | Common Stock
Class of Stock [Line Items]
Shelf registration statement, aggregate dollar amount | $					$ 96,200
Cantor Fitzgerald and Co. | Common Stock
Class of Stock [Line Items]
Proceeds from sale of common stock | $		$ 71,000
New shares issued under shelf registration (in shares)		2,500,000
Price per share issued under underwriting agreement (in dollars per share) | $ / shares		$ 29.50
February 2019 | Common Stock
Class of Stock [Line Items]
Warrants outstanding (in shares)			75,000
Subsequent Event | February 2019 | Common Stock
Class of Stock [Line Items]
Warrants outstanding (in shares)						75,000