Accumulated Other Comprehensive Income (Loss) Reclassification Adjustments (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Interest expense		$ (27.7)	$ (32.4)	$ (90.7)	$ (68.2)
Investment Income, Nonoperating		4.3	5.3	15.8	16.5
Total reclassifications for the period		1.0	(0.1)	(2.4)	1.7
Interest Rate Hedges | Reclassification out of Accumulated Other Comprehensive Income [Member]
Interest expense		0.0	0.0	(0.4)	(0.1)
Net actuarial loss amortization | Reclassification out of Accumulated Other Comprehensive Income [Member]
Net periodic benefit cost	[1]	1.0	(0.1)	(2.0)	1.8
Net actuarial loss amortization, tax		(0.5)	(0.3)	0.5	(1.3)
Pension and Other Post-retirement Benefits
Total reclassifications for the period		$ 1.0	$ (0.1)	$ (2.0)	$ 1.8

[1] Pension and other post-retirement benefits AOCI components are included in the computation of net periodic benefit cost (refer to note 14).