Stockholders' Equity - Other Offerings (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	9 Months Ended		23 Months Ended
	Aug. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017
Subsidiary, Sale of Stock [Line Items]
Common stock, shares outstanding		30,991,060		30,991,060	30,856,505
Underwriters' discounts and commissions	$ 5,176
At The Market Offering
Subsidiary, Sale of Stock [Line Items]
Number of shares issued		0
Common stock, shares outstanding		635,000		635,000
Issuance price (in dollars per share)				$ 31.50
Aggregate gross proceeds				$ 20,003
Expenses related to stock issuance		$ 691
Net offering proceeds			$ 19,311
Public Offering August 2017
Subsidiary, Sale of Stock [Line Items]
Number of shares issued	3,747,602
Issuance price (in dollars per share)	$ 23.02
Aggregate gross proceeds	$ 86,270
Expenses related to stock issuance	176
Net offering proceeds	$ 80,918		$ 80,918