Stockholders' Equity - At The Market Issuances (Details) - At-The-Market Issuance Sales Agreement - USD ($) $ in Millions	1 Months Ended	6 Months Ended	12 Months Ended	33 Months Ended
	Oct. 31, 2015	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2018
Stockholders' Equity
Number of shares sold by the entity		0	215,539	597,356
Value of shares sold by the entity				$ 9.6
Net proceeds after deducting commissions and other transactions costs				9.4
Remaining reserved amount under shelf registration		$ 90.4		$ 90.4
Cowen & Co. LLC | Maximum
Stockholders' Equity
Maximum aggregate offering price	$ 100.0
Percentage of gross proceeds payable as commission	3.00%
Notice period of termination of sales agreement	10 days