Accumulated Other Comprehensive (Loss) Income, Reclassifications out of Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Interest expense		$ 140.7	$ 170.3	$ 162.9
Total before tax		(112.7)	(63.2)	(366.7)
Income tax benefit		131.2	31.9	14.7
Net of tax		18.5	(31.3)	$ (352.0)
Reclassification out of Accumulated Other Comprehensive (Loss) Income [Member]
Income Statement [Abstract]
Net of tax		14.6	9.1
Loss on Cash Flow Hedges - Interest Rate Swaps [Member] | Reclassification out of Accumulated Other Comprehensive (Loss) Income [Member]
Income Statement [Abstract]
Interest expense		18.5	11.6
Total before tax		18.5	11.6
Income tax benefit		(7.0)	(4.4)
Net of tax		11.5	7.2
Amortization of Defined Benefit Pension and Other Postretirement Benefit Items [Member] | Reclassification out of Accumulated Other Comprehensive (Loss) Income [Member]
Income Statement [Abstract]
Net periodic benefit cost	[1]	5.0	3.0
Total before tax		5.0	3.0
Income tax benefit		(1.9)	(1.1)
Net of tax		$ 3.1	$ 1.9

[1] These components are included in the computation of net periodic benefit cost (see Note 11 "Benefit Plans" for additional details).