Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2020		Sep. 30, 2019		Sep. 30, 2020		Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period					$ 6,996.2
Income Tax Expense (Benefit)	$ 24.0		$ 28.0		49.1		$ 57.7
Accumulated other comprehensive loss at end of period	7,510.7				7,510.7
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(23.5)		(24.7)		(24.1)		(25.3)
Net current period other comprehensive loss	0.3		0.3		0.9		0.9
Accumulated other comprehensive loss at end of period	(23.2)		(24.4)		(23.2)		(24.4)
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)	0.1		0.1		0.3		0.3
Gains and Losses on Cash Flow Hedges | Interest Rate Swap
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Losses reclassified from net accumulated other comprehensive loss	(0.3)	[1]	(0.3)	[1]	(0.9)	[2]	(0.9)	[2]
Defined Benefit and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(2.5)		(0.2)		(2.5)		(0.2)
Net current period other comprehensive loss	0.0		0.0		0.0		0.0
Accumulated other comprehensive loss at end of period	(2.5)		(0.2)		(2.5)		(0.2)
Defined Benefit 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
Defined Benefit and Postretirement Items | Interest Rate Swap
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Losses reclassified from net accumulated other comprehensive loss	0.0	[1]	0.0	[1]	0.0	[2]	0.0	[2]
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(26.0)		(24.9)		(26.6)		(25.5)
Net current period other comprehensive loss	0.3		0.3		0.9		0.9
Accumulated other comprehensive loss at end of period	(25.7)		(24.6)		(25.7)		(24.6)
Total | Interest Rate Swap
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Losses reclassified from net accumulated other comprehensive loss	$ (0.3)	[1]	$ (0.3)	[1]	$ (0.9)	[2]	$ (0.9)	[2]

[1] Included in interest charges.

[2] Included in interest charges.