CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income (loss)		$ 3.2	$ (97.3)	$ (1,011.6)
Other comprehensive income, net of tax:
Net income (loss)		3.2	(97.3)	(1,011.6)
Pension and other postretirement plans adjustments:
Current period prior service cost(1)	[1]	0.0	0.0	(0.1)
Current period net actuarial gain (loss)(2)	[2]	(1.8)	1.1	(4.2)
Amortization of prior service cost(3)	[3]	0.0	(0.3)	0.4
Amortization of net actuarial (gain) loss(4)	[4]	0.7	0.4	0.6
Net curtailment and settlement cost incurred(5)	[5]	0.8	0.6	0.1
Other comprehensive income (loss)		(0.3)	1.8	(3.2)
Comprehensive income (loss)		$ 2.9	$ (95.5)	$ (1,014.8)

[1] Presented net of income tax benefit of $0.1 million for the year ended December 31, 2018.

[2] Presented net of income tax benefit of $0.5 million for the year ended December 31, 2020, net of income tax expense of $0.3 million for the year ended December 31, 2019 and net of income tax benefit of $1.3 million for the year ended December 31, 2018.

[3] Presented net of income tax benefit of $0.1 million for the year ended December 31, 2019 and net of income tax expense of $0.1 million for the year ended December 31, 2018.

[4] Presented net of income tax expense of $0.2 million, $0.1 million and $0.2 million for the years ended December 31, 2020, 2019 and 2018, respectively.

[5] Presented net of income tax expense $0.2 million and $0.2 million for the years ended December 31, 2020 and 2019, respectively.