Common Stock and Related Matters - Additional Information (Details) - USD ($)	1 Months Ended	3 Months Ended		12 Months Ended
	Apr. 30, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	May 09, 2017
Securities Financing Transaction [Line Items]
Proceeds from issuance of common stock		$ 58,100,000	$ 664,800,000
Series A Warrants [Member]
Securities Financing Transaction [Line Items]
Warrants exercised (in shares)		99,888		19,983,843
Warrants remaining					99,888
Common Stock [Member]
Securities Financing Transaction [Line Items]
Issuance of common stock (in shares)		1,163,000	11,886,000
Exercise of warrants - share settled (in shares)		58,814		11,336,856
Equity Distribution Agreement December 2016 [Member]
Securities Financing Transaction [Line Items]
Issuance of common stock (in shares)		1,162,963
Proceeds from issuance of common stock		$ 57,700,000
Remaining amount of common stock available under equity distribution agreement		$ 266,000,000
Equity Distribution Agreement May 2017 [Member]
Securities Financing Transaction [Line Items]
Issuance of common stock (in shares)		0
Amount of common stock authorized under equity distribution agreement						$ 750,000,000
Equity Distribution Agreement May 2017 [Member] | Subsequent Event [Member]
Securities Financing Transaction [Line Items]
Issuance of common stock (in shares)	640,228
Proceeds from issuance of common stock	$ 29,000,000
Remaining amount of common stock available under equity distribution agreement	$ 720,800,000