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	Jun. 30, 2021	Dec. 31, 2020	Oct. 29, 2020	Dec. 31, 2019
6.875% senior secured notes due November 1, 2027
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of long-term debt	$ 477.5	$ 476.9
Interest rate (as a percent)	6.875%		6.875%
5.50% convertible senior notes due March 1, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of long-term debt	$ 0.0	$ 83.6		$ 84.2
Interest rate (as a percent)	5.50%	5.50%
Fair Value | 6.875% senior secured notes due November 1, 2027
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of long-term debt	$ 531.7	$ 532.3
Fair Value | 5.50% convertible senior notes due March 1, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of long-term debt	$ 0.0	$ 169.8