Stockholders' Equity and Derivative Liability - Warrants - Controlled Equity Offering Sales Agreement (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended
	Mar. 09, 2015	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Controlled equity offering sales agreement
Common stock, par value (in dollars per share)		$ 0.0001		$ 0.0001
Controlled Equity Offering Sales Agreement
Controlled equity offering sales agreement
Common stock, par value (in dollars per share)	$ 0.0001
Maximum aggregate offering price	$ 50.0
Selling agent fee as a percentage of gross sales price per share sold	3.00%
Sale of shares (in shares)		0	0