Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2020		Jun. 30, 2019		Jun. 30, 2020		Jun. 30, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive (loss) income at beginning of period					$ 6,081.8
Income Tax Expense (Benefit)	$ (7.1)		$ 1.7		(12.7)		$ 7.8
Accumulated other comprehensive (loss) income at end of period	6,289.8				6,289.8
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive (loss) income at beginning of period	(19.2)		(20.0)		(19.4)		(20.2)
(Gains) losses reclassified from net accumulated other comprehensive loss	0.0	[1]	0.0	[1]	0.0	[2]	0.0	[2]
Other Comprehensive Income (Loss), Net of Tax	0.2		0.2		0.4		0.4
Accumulated other comprehensive (loss) income at end of period	(19.0)		(19.8)		(19.0)		(19.8)
Gains and Losses on Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Income Tax Expense (Benefit)	0.0		0.1		0.1		0.1
Reclassification from AOCI, Current Period, Tax	0.0		0.0		0.0		0.0
Gains and Losses on Cash Flow Hedges | Interest Rate Swap [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
(Gains) losses reclassified from net accumulated other comprehensive loss	(0.2)	[3]	(0.2)	[3]	(0.4)	[4]	(0.4)	[4]
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive (loss) income at beginning of period	(3.1)		(2.9)		(3.1)		(2.9)
(Gains) losses reclassified from net accumulated other comprehensive loss	(0.1)	[1]	0.0	[1]	(0.1)	[2]	0.0	[2]
Other Comprehensive Income (Loss), Net of Tax	0.1		0.0		0.1		0.0
Accumulated other comprehensive (loss) income at end of period	(3.0)		(2.9)		(3.0)		(2.9)
Defined Benefit Pension and Postretirement Items | Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Income Tax Expense (Benefit)	0.0		0.0		0.0		0.0
Reclassification from AOCI, Current Period, Tax	0.0		0.0		0.0		0.0
Defined Benefit Pension and Postretirement Items | Interest Rate Swap [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
(Gains) losses reclassified from net accumulated other comprehensive loss	0.0	[3]	0.0	[3]	0.0	[4]	0.0	[4]
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive (loss) income at beginning of period	(22.3)		(22.9)		(22.5)		(23.1)
(Gains) losses reclassified from net accumulated other comprehensive loss	(0.1)	[1]	0.0	[1]	(0.1)	[2]	0.0	[2]
Other Comprehensive Income (Loss), Net of Tax	0.3		0.2		0.5		0.4
Accumulated other comprehensive (loss) income at end of period	(22.0)		(22.7)		(22.0)		(22.7)
Total | Interest Rate Swap [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
(Gains) losses reclassified from net accumulated other comprehensive loss	$ (0.2)	[3]	$ (0.2)	[3]	$ (0.4)	[4]	$ (0.4)	[4]

[1] Included in the computation of net periodic pension and postretirement benefit costs.

[2] Included in the computation of net periodic pension and postretirement benefit costs.

[3] Included in interest charges.

[4] Included in interest charges.