Derivative Financial Instruments - 6% Convertible Promissory Notes (Additional Information) (Details) - 6% Convertible Promissory Notes - Convertible promissory notes - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 10, 2016	Jan. 29, 2016	Dec. 15, 2015
Debt Instrument [Line Items]
Interest rate percentage	6.00%		6.00%
Debt face amount					$ 75,000,000	$ 25,000,000	$ 160,000,000
Convertible stock price (in dollars per share)	$ 11.25		$ 11.25		$ 20.61		$ 11.25
Convertible debt, stock price trigger			75.00%
Gain (loss) on embedded derivative	$ 0	$ (23,500,000)	$ 0	$ (31,000,000)
Embedded derivative liability	$ 177,200,000		$ 177,200,000		$ 177,200,000