CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income (loss)		$ (97.3)	$ (1,011.6)	$ 269.3
Other comprehensive income, net of tax:
Future tax effective rate change(1)	[1]	0.0	0.0	(3.8)
Pension and other postretirement plans adjustments:
Current period prior service cost(2)	[2]	0.0	(0.1)	2.4
Current period net actuarial (gain) loss(3)	[3]	1.1	(4.2)	5.8
Amortization of prior service cost(4)	[4]	(0.3)	0.4	0.5
Amortization of net actuarial (gain) loss(5)	[5]	0.4	0.6	0.3
Net curtailment and settlement cost incurred(6)	[6]	0.6	0.1	0.4
Other comprehensive income (loss)		1.8	(3.2)	5.6
Comprehensive income (loss)		$ (95.5)	$ (1,014.8)	$ 274.9

[1] Refer to Recent Accounting Developments in Note 1 – Summary of Significant Accounting Policies.

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

[3] Presented net of income tax expense of $0.3 million for the year ended December 31, 2019, net of income tax benefit of $1.3 million for the year ended December 31, 2018 and net of income tax expense of $1.8 million for the year ended December 31, 2017.

[4] Presented net of income tax benefit of $0.1 million for the year ended December 31, 2019, net of income tax expense of $0.1 million and $0.2 million for the years ended December 31, 2018 and 2017, respectively.

[5] Presented net of income tax expense of $0.1 million, $0.2 million and $0.1 million for the years ended December 31, 2019, 2018 and 2017, respectively.

[6] Presented net of income tax expense $0.2 million for the year ended December 31, 2019 and net of income tax expense of $0.1 million for the year ended December 31, 2017.