Common Stock and Related Matters - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Sep. 20, 2018	May 09, 2017
Securities Financing Transaction [Line Items]
Proceeds from issuance of common stock			$ 576,500,000	$ 1,573,400,000
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)			11,376,000	30,721,000
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)			6,315,711
Proceeds from issuance of common stock			$ 318,600,000
Equity Distribution Agreement May 2017 [Member]
Securities Financing Transaction [Line Items]
Issuance of common stock (in shares)			5,060,817
Proceeds from issuance of common stock			$ 253,400,000
Amount of common stock authorized under equity distribution agreement							$ 750,000,000
Remaining amount of common stock available under equity distribution agreement	$ 494,600,000		$ 494,600,000
Equity Distribution Agreement September 2018 [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