Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended	2 Months Ended	12 Months Ended
	Jan. 23, 2018	May 25, 2017	Jan. 31, 2018	Feb. 26, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsequent Event [Line Items]
Proceeds from sale of common stock, net					$ 134,215	$ 167,383	$ 50,451
Common Stock
Subsequent Event [Line Items]
Common stock issued and sold under sales agreement (in shares)					1,100,000	4,200,000	1,754,556
Proceeds from sale of common stock, net		$ 72,700			$ 61,100		$ 50,400
New shares issued under shelf registration (in shares)		1,400,000
Price per share issued under underwriting agreement (in dollars per share)		$ 53.75
Common Stock | Subsequent Event
Subsequent Event [Line Items]
Common stock issued and sold under sales agreement (in shares)			1,000,000
Proceeds from sale of common stock, net			$ 62,300	$ 136,200
New shares issued under shelf registration (in shares)	1,300,000
Proceeds from issuance of common stock, net of underwriting discounts, fees and expenses			$ 74,000