Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Foreign currency translation
Balance at beginning of period		$ (0.6)	$ 1.1	$ (3.6)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax [Abstract]
Foreign currency translation gain (loss)		(0.6)	(1.7)	4.7
Balance at end of period		(1.2)	(0.6)	1.1
Cash flow hedges
Balance at beginning of period		(14.6)	7.2	22.3	$ 39.9
Cumulative effect of adopting new hedging standard			0.0	0.8	0.0
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax [Abstract]
Gains (losses) arising in period		(31.5)	(5.6)	(11.5)
Income tax expense (benefit)		(1.1)	0.0	0.0
Gains (losses) arising in period, net of tax		(30.4)	(5.6)	(11.5)
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, before Tax [Abstract]
Reclassification of losses (gains) to net income		8.9	(10.3)	(6.1)
Income tax (expense) benefit (b)	[1]	0.3	0.0	0.0
Net amount of reclassification of losses (gains) to net income (loss), net of tax		8.6	(10.3)	(6.1)
Total other comprehensive income (loss), net of tax		(21.8)	(15.9)	(17.6)
Balance at end of period		(14.6)	7.2	22.3	$ 39.9
Pension and OPEB Plans
Balance at beginning of period		(106.6)	(73.6)	(125.0)
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax [Abstract]
Prior service credit (cost) arising in period		0.0	11.1	4.7
Gains (losses) arising in period		50.5	(63.6)	94.2
Other Comprehensive (Income) Loss, Defined Benefit Plan, before Reclassification Adjustment and Tax		50.5	(52.5)	98.9
Income tax expense (benefit)		2.0	0.0	0.0
Gains (losses) arising in period, net of tax		48.5	(52.5)	98.9
Other Comprehensive Income (Loss), Reclassification Adjustment, Pension and Other Postretirement Benefit Plans, Included in Net Income, before Tax [Abstract]
Reclassification of prior service cost (credits) included in net income		(9.6)	(9.8)	(53.8)
Reclassification of actuarial (gains) losses included in net income (loss)		15.7	29.3	6.3
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, before Tax		6.1	19.5	(47.5)
Income tax (expense) benefit	[1]	0.2	0.0	0.0
Amount of reclassification to net income (loss), net of tax		5.9	19.5	(47.5)
Total other comprehensive income (loss), net of tax		54.4	(33.0)	51.4
Balance at end of period		(52.2)	(106.6)	(73.6)
Pension and OPEB Expense (Income) [Member]
Other Comprehensive Income (Loss), Reclassification Adjustment, Pension and Other Postretirement Benefit Plans, Included in Net Income, before Tax [Abstract]
Reclassification of prior service cost (credits) included in net income	[2]	(9.6)	(9.8)	(53.8)
Reclassification of actuarial (gains) losses included in net income (loss)	[2]	$ 15.7	$ 29.3	$ 6.3

[1] Included in income tax expense (benefit)

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