Equity - Additional Information (Details) - USD ($)	1 Months Ended		12 Months Ended
	Jul. 31, 2017	Jul. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stockholders Equity [Line Items]
Net proceeds from sale of common stock					$ 461,136,000
June 2010 Common Stock Warrant
Stockholders Equity [Line Items]
Common stock warrants outstanding			149,700	149,700
Common stock warrants, issued date			2010-06	2010-06
Common stock warrants, contractual term			10 years	10 years
Common stock warrants, exercise price			$ 3.01	$ 3.01
June 2011 Common Stock Warrant
Stockholders Equity [Line Items]
Common stock warrants outstanding			149,700	149,700
Common stock warrants, issued date			2011-06	2011-06
Common stock warrants, contractual term			10 years	10 years
Common stock warrants, exercise price			$ 3.01	$ 3.01
ATM Sales Agreement
Stockholders Equity [Line Items]
Option to sell common stock for cash		$ 150,000,000
Common stock shares sold			912,351	1,159,415
Net proceeds from sale of common stock			$ 67,600,000	$ 79,500,000
Additional ATM Sales Agreement
Stockholders Equity [Line Items]
Common stock shares sold			1,338,866
Net proceeds from sale of common stock			$ 64,300,000
Additional ATM Sales Agreement | Maximum
Stockholders Equity [Line Items]
Option to sell common stock for cash	$ 150,000,000