Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Foreign currency translation
Balance at beginning of period		$ 1.1	$ (3.6)
Other comprehensive income (loss)—foreign currency translation gain (loss)		1.1	0.3
Balance at end of period		2.2	(3.3)
Cash flow hedges
Balance at beginning of period		22.3	39.9
Cumulative effect of adopting new hedging standard				$ 0.0
Other comprehensive income (loss):
Gains (losses) arising in period		(1.4)	(8.0)
Income tax expense (benefit) (b)		0.0	0.0
Gains (losses) arising in period, net of tax		(1.4)	(8.0)
Reclassification of losses (gains) to net income (loss)		(7.6)	(0.6)
Income tax (expense) benefit (b)	[1]	0.0	0.0
Net amount of reclassification of losses (gains) to net income, net of tax		(7.6)	(0.6)
Total other comprehensive income (loss), net of tax		(9.0)	(8.6)
Balance at end of period		14.1	31.3
Pension and OPEB plans
Balance at beginning of period		(73.6)	(125.0)
Reclassification to net income:
Prior service costs (credits) (a)		(2.4)	(13.4)
Actuarial (gains) losses (a)		3.6	1.6
Subtotal		1.2	(11.8)
Income tax (expense) benefit (b)	[1]	0.0	0.0
Amount of reclassification to net income, net of tax		1.2	(11.8)
Total other comprehensive income (loss), net of tax		1.2	(11.8)
Balance at end of period		(72.4)	(136.8)
Cost of Sales [Member]
Other comprehensive income (loss):
Reclassification of losses (gains) to net income (loss)		(7.6)	(0.6)
Pension and OPEB Expense (Income) [Member]
Reclassification to net income:
Prior service costs (credits) (a)	[2]	(2.4)	(13.4)
Actuarial (gains) losses (a)	[2]	$ 3.6	$ 1.6
Accumulated Other Comprehensive Income (Loss) [Member]
Cash flow hedges
Cumulative effect of adopting new hedging standard				$ 0.8	$ 0.0

[1] Included in income tax expense (benefit)

[2] Included in pension and OPEB (income) expense