Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Foreign currency translation
Balance at beginning of period		$ (1.0)	$ (1.7)	$ (3.6)	$ (2.1)
Other comprehensive income (loss)—foreign currency translation gain (loss)		1.4	0.1	4.0	0.5
Balance at end of period		0.4	(1.6)	0.4	(1.6)
Cash flow hedges
Balance at beginning of period		24.9	21.4	46.4	(34.0)
Other comprehensive income (loss):
Gains (losses) arising in period		7.3	2.4	(8.9)	35.3
Income tax expense (benefit)		0.0	(2.9)	0.0	0.0
Gains (losses) arising in period, net of tax		7.3	5.3	(8.9)	35.3
Reclassification of losses (gains) to net income (loss)		(1.9)	(1.4)	(7.2)	26.3
Income tax expense (benefit)		0.0	(2.3)	0.0	0.0
Net amount of reclassification of losses (gains) to net income (loss)		(1.9)	0.9	(7.2)	26.3
Total other comprehensive income (loss), net of tax		5.4	6.2	(16.1)	61.6
Balance at end of period		30.3	27.6	30.3	27.6
Pension and OPEB plans
Balance at beginning of period		(130.0)	(165.5)	(106.3)	(151.1)
Other comprehensive income (loss):
Prior service credit (cost) arising in period		0.0	(8.9)	0.0	(8.9)
Gains (losses) arising in period		0.0	(12.8)	0.0	(12.8)
Subtotal		0.0	(21.7)	0.0	(21.7)
Income tax expense (benefit)		0.0	0.0	0.0	0.0
Gains (losses) arising in period, net of tax		0.0	(21.7)	0.0	(21.7)
Reclassification to net income (loss):
Prior service costs (credits) (b)		(13.6)	(13.5)	(40.6)	(41.1)
Actuarial (gains) losses (b)		1.6	11.8	4.9	23.7
Subtotal		(12.0)	(1.7)	(35.7)	(17.4)
Income tax expense (benefit)		0.0	1.3	0.0	0.0
Amount of reclassification to net income (loss), net of tax		(12.0)	(3.0)	(35.7)	(17.4)
Total other comprehensive income (loss), net of tax		(12.0)	(24.7)	(35.7)	(39.1)
Balance at end of period		(142.0)	(190.2)	(142.0)	(190.2)
Cost of Sales [Member]
Other comprehensive income (loss):
Reclassification of losses (gains) to net income (loss)	[1]	(1.9)	(1.4)	(7.2)	26.3
Pension and OPEB Expense (Income) [Member]
Reclassification to net income (loss):
Prior service costs (credits) (b)	[2]	(13.6)	(13.5)	(40.6)	(41.1)
Actuarial (gains) losses (b)	[2]	$ 1.6	$ 11.8	$ 4.9	$ 23.7

[1] Included in cost of products sold.

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