Financial Instruments and Fair Value Measurements - Fair Values of Financial Instruments Not Measured at Fair Value in Consolidated Balance Sheets (Details) - Senior Notes - USD ($) $ in Millions	Sep. 30, 2019	Aug. 06, 2019	Dec. 31, 2018	Dec. 31, 2016
10.75% senior secured notes due 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of long-term debt	$ 433.9		$ 432.0
Interest rate	10.75%
5.50% convertible senior notes due 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of long-term debt	$ 79.1	$ 84.2	194.2
Interest rate	5.50%			5.50%
Fair Value | 10.75% senior secured notes due 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of long-term debt	$ 481.3		486.8
Fair Value | 5.50% convertible senior notes due 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of long-term debt	$ 92.9		$ 298.5