Debt - Estimated Fair Values of Long-term Debt (Details) - Senior Notes - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
10.75% senior secured notes due April 15, 2022
Debt Instrument [Line Items]
Interest rate	10.75%
5.50% convertible senior notes due March 1, 2021
Debt Instrument [Line Items]
Interest rate	5.50%		5.50%
Estimated Fair Value | 10.75% senior secured notes due April 15, 2022
Debt Instrument [Line Items]
Long-term debt	$ 486.8	$ 492.8
Estimated Fair Value | 5.50% convertible senior notes due March 1, 2021
Debt Instrument [Line Items]
Long-term debt	$ 298.5	$ 237.9