Derivative Financial Instruments - 6% Convertible Promissory Notes (Additional Information) (Details) - 6% Convertible Promissory Notes - Convertible promissory notes - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Jan. 31, 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)	$ 20.61					$ 11.25
Convertible debt, stock price trigger	75.00%
Gain (loss) on embedded derivative	$ 0	$ (7,500,000)
Embedded derivative liability				$ 177,200,000