Debt - Convertible Notes (Details) $ / shares in Units, shares in Millions			1 Months Ended	3 Months Ended
	Oct. 02, 2018 USD ($) $ / shares shares	Sep. 21, 2017 USD ($) shares	Feb. 28, 2015 USD ($) $ / shares shares	Mar. 31, 2019 USD ($) $ / shares shares	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) $ / shares shares
Debt Instrument [Line Items]
Non-cash component of warrant exercise				$ 0	$ 17,760,000
Common stock, par value (in usd per share) | $ / shares				$ 0.01		$ 0.01
Interest expense				$ 9,962,000	10,626,000
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%
Shares issued upon conversion (in shares) | shares			19.6
Conversion ratio				0.1958
Conversion price (in usd per share) | $ / shares			$ 5.11
Non-cash component of warrant exercise	$ 12,900,000	$ 80,000,000
Shares issued (in shares) | shares	2.6	16.8
Common stock, par value (in usd per share) | $ / shares	$ 0.01
Aggregate number of shares with convertible beneficial conversion feature (in shares) | shares	0.1	1.1
Induced conversion of convertible debt expense	$ 900,000	$ 13,000,000
Outstanding balance on debt conversion				$ 7,100,000		$ 7,100,000
Shares convertible into common shares (in shares) | shares				1.4		1.4
Interest expense				$ 100,000	$ 200,000