Debt - Schedule of Components of Long-term Debt (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2016
Debt Instrument [Line Items]
Finance leases	$ 5,400,000
Finance leases		$ 5,800,000
Other debt	40,200,000	20,800,000
Total	674,400,000	652,800,000
Less – current maturities	7,300,000	10,000,000
Total long-term debt	$ 667,100,000	642,800,000
5.50% convertible senior notes due March 1, 2021
Debt Instrument [Line Items]
Face value			$ 1,000
Senior Notes | 10.75% senior notes due April 15, 2022
Debt Instrument [Line Items]
Interest rate	10.75%
Face value	$ 440,000,000	440,000,000
Unamortized discount and fees	7,400,000	8,000,000
Senior notes	$ 432,600,000	432,000,000
Senior Notes | 5.50% convertible senior notes due March 1, 2021
Debt Instrument [Line Items]
Interest rate	5.50%		5.50%
Face value	$ 213,500,000	213,500,000	$ 213,500,000
Unamortized discount and fees	17,300,000	19,300,000
Senior notes	$ 196,200,000	$ 194,200,000