Accumulated Other Comprehensive Loss Reclassification Adjustments (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Interest Expense		$ (8.5)	$ (8.3)	$ (24.1)	$ (23.1)
Investment income		5.9	7.9	20.6	26.6
Total reclassifications for the period		1.0	0.4	3.1	0.8
Interest Rate Hedges
Total reclassifications for the period		(0.1)	(0.1)	(0.2)	(0.2)
Interest Rate Hedges | Reclassification out of Accumulated Other Comprehensive Income [Member]
Interest Expense		(0.1)	(0.1)	(0.2)	(0.2)
Net actuarial loss amortization | Reclassification out of Accumulated Other Comprehensive Income [Member]
Net periodic benefit cost	[1]	1.1	0.6	3.4	1.6
Net actuarial loss amortization, tax		(0.6)	(0.3)	(1.9)	(0.9)
Net prior service benefit amortization | Reclassification out of Accumulated Other Comprehensive Income [Member]
Net periodic benefit cost	[1]	0.0	(0.1)	(0.1)	(0.2)
Net prior service benefit amortization, amount recognized in other comprehensive income, tax		0.0	0.0	0.0	(0.1)
Pension and Other Post-retirement Benefits
Total reclassifications for the period		1.1	0.5	3.3	1.4
Pension and Other Post-retirement Benefits | Reclassification out of Accumulated Other Comprehensive Income [Member]
Net periodic benefit cost		1.1	0.5	3.3	1.4
Unrealized loss on securities
Investment income			0.0		(0.4)
Unrealized loss on securities | Reclassification out of Accumulated Other Comprehensive Income [Member]
Investment income		0.0		0.0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax		$ 0.0	$ 0.0	$ 0.0	$ 0.2

[1] Pension and other post-retirement benefits AOCI components are included in the computation of net periodic benefit cost (refer to note 12).