Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020		Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive (loss) income at beginning of period	$ 6,081.8
Accumulated other comprehensive (loss) income at end of period	6,168.6
Income Tax Expense (Benefit)	(5.6)		$ 6.0
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	(19.4)		(20.2)
Other Comprehensive Income (Loss), Net of Tax	0.2		0.2
Accumulated other comprehensive (loss) income at end of period	(19.2)		(20.0)
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]
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax	0.0		0.0
Income Tax Expense (Benefit)	0.1		0.1
Reclassification from AOCI, Current Period, Tax	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)	[1]	(0.2)	[2]
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)
Other Comprehensive Income (Loss), Net of Tax	0.0		0.0
Accumulated other comprehensive (loss) income at end of period	(3.1)		(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]
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax	0.0		0.0
Income Tax Expense (Benefit)	0.0		0.0
Reclassification from AOCI, Current Period, Tax	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	[1]	0.0	[2]
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive (loss) income at beginning of period	(22.5)		(23.1)
Other Comprehensive Income (Loss), Net of Tax	0.2		0.2
Accumulated other comprehensive (loss) income at end of period	(22.3)		(22.9)
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)	[1]	$ (0.2)	[2]

[1] Included in interest charges.

[2] Included in interest charges.