CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income (loss)		$ 269.3	$ (1,245.0)	$ (149.4)
Other comprehensive income, net of tax:
Future tax effective rate change(1)	[1]	(3.8)	0.0	0.0
Pension and other postretirement plans adjustments:
Current period prior service cost(2)	[2]	(2.4)	0.0	0.6
Current period net actuarial (gain) loss(3)	[3]	5.8	(5.6)	(0.5)
Amortization of prior service cost(4)	[4]	0.5	0.8	8.2
Amortization of net actuarial (gain) loss(5)	[5]	0.3	0.5	0.3
Net curtailment and settlement cost incurred(6)	[6]	0.4	0.0	4.5
Other comprehensive income (loss)		5.6	(4.3)	11.9
Comprehensive income (loss)		$ 274.9	$ (1,249.3)	$ (137.5)

[1] The new tax legislation changed the federal corporate income tax rate from 35% to 21% starting in 2018. The rate change caused the Company to revalue its deferred tax liabilities and assets using the lower rate.

[2] Presented net of income tax expense of $0.8 million for the year ended December 31, 2017 and net of income tax benefit of $0.3 million for the year ended December 31, 2015.

[3] Presented net of income tax expense of $1.8 million for the year ended December 31, 2017 and net of income tax benefit of $3.3 million and $0.3 million for the years ended December 31, 2016 and 2015, respectively.

[4] Presented net of income tax expense of $0.2 million, $0.5 million, and $4.9 million for the years ended December 31, 2017, 2016, and 2015, respectively.

[5] Presented net of income tax expense of $0.1 million, $0.3 million, and $0.2 million for the years ended December 31, 2017, 2016, and 2015, respectively.

[6] Presented net of income tax expense of $0.1 million and $2.6 million for the years ended December 31, 2017 and 2015, respectively.