Debt - Convertible Notes (Details) $ / shares in Units, shares in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Feb. 28, 2015 USD ($)	Jun. 30, 2019 USD ($) $ / shares	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) $ / shares	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) $ / shares shares
Debt Instrument [Line Items]
Non-cash component of warrant exercise				$ 0	$ 137,979,000
Common stock, par value (in usd per share) | $ / shares		$ 0.01		$ 0.01		$ 0.01
Outstanding balance on debt conversion		$ 7,100,000		$ 7,100,000		$ 1,400,000
Interest expense				$ 20,373,000	19,790,000
Convertible Senior Notes
Debt Instrument [Line Items]
Face amount	$ 100,000,000.0
Proceeds from debt, net of issuance costs	96,300,000
Payments of debt issuance costs	$ 3,700,000
Stated interest rate	4.75%
Effective interest rate		5.48%		5.48%
Conversion ratio				0.1958
Outstanding balance on debt conversion						$ 7,100,000
Shares convertible into common shares (in shares) | shares						1.4
Interest expense		$ 100,000	$ 200,000	$ 200,000	$ 500,000