Accumulated Other Comprehensive Income (Loss) Reclassification Adjustments (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Document Period End Date				Jun. 30, 2018
Interest expense		$ (28.4)	$ (32.2)	$ (54.4)	$ (63.0)
Investment Income, Nonoperating		1.9	5.1	5.4	11.5
Total reclassifications for the period		2.6	0.8	4.8	(3.4)
Interest Rate Hedges | Reclassification out of Accumulated Other Comprehensive Income [Member]
Interest expense		0.8	(0.1)	1.2	(0.4)
Net actuarial loss amortization | Reclassification out of Accumulated Other Comprehensive Income [Member]
Net periodic benefit cost	[1]	1.8	0.9	3.6	(3.0)
Net actuarial loss amortization, tax				(0.2)	1.0
Pension and Other Post-retirement Benefits
Total reclassifications for the period		$ 1.8	$ 0.9	$ 3.6	$ (3.0)

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