Accumulated Other Comprehensive Income (Loss) Reclassification Adjustments (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Document Period End Date				Sep. 30, 2018
Interest expense		$ (45.2)	$ (27.7)	$ (99.6)	$ (90.7)
Investment Income, Nonoperating		2.2	4.3	7.6	15.8
Total reclassifications for the period		(1.0)	1.0	3.7	(2.4)
Interest Rate Hedges | Reclassification out of Accumulated Other Comprehensive Income [Member]
Interest expense		1.0	0.0	2.1	(0.4)
Net actuarial loss amortization | Reclassification out of Accumulated Other Comprehensive Income [Member]
Net periodic benefit cost	[1]	(2.0)	1.0	1.6	(2.0)
Net actuarial loss amortization, tax		1.0	(0.5)	0.8	0.5
Pension and Other Post-retirement Benefits
Total reclassifications for the period		$ (2.0)	$ 1.0	$ 1.6	$ (2.0)

[1] Pension and other post-retirement benefits AOCI components are included in the computation of net periodic benefit cost (refer to note 16).