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 at beginning of period					$ 6,996.2
Income taxes	$ 7.7		$ 10.6		25.1		$ 29.7
Accumulated other comprehensive loss at end of period	7,420.1				7,420.1
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	(23.8)		(25.0)		(24.1)		(25.3)
Net current period other comprehensive loss	0.3		0.3		0.6		0.6
Accumulated other comprehensive loss at end of period	(23.5)		(24.7)		(23.5)		(24.7)
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 taxes	0.1		0.1		0.2		0.1
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]	(0.6)	[2]	(0.6)	[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 taxes	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	(26.3)		(25.2)		(26.6)		(25.5)
Net current period other comprehensive loss	0.3		0.3		0.6		0.6
Accumulated other comprehensive loss at end of period	(26.0)		(24.9)		(26.0)		(24.9)
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]	$ (0.6)	[2]	$ (0.6)	[2]

[1] Included in interest charges.

[2] Included in interest charges.