Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period				$ 6,298.2
Accumulated other comprehensive loss at end of period		$ 7,005.6		7,005.6
Interest rate derivatives, tax		28.0	$ 40.7	57.7	$ 115.0
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		(24.7)	(25.8)	(25.3)	(26.4)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		0.0	0.0	0.0	0.0
Losses reclassified from net accumulated other comprehensive loss		0.0	0.0	0.0	0.0
Net current period other comprehensive loss		0.3	0.3	0.9	0.9
Accumulated other comprehensive loss at end of period		(24.4)	(25.5)	(24.4)	(25.5)
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), tax		0.0	0.0	0.0	0.0
Interest rate derivatives, tax		0.1	0.1	0.3	0.3
Amortization of net actuarial loss, tax		0.0	0.0	0.0	0.0
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	[1]	(0.3)	(0.3)	(0.9)	(0.9)
Defined Benefit and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		(0.2)	(0.3)	(0.2)	(0.3)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		0.0	(0.2)	0.0	(0.2)
Losses reclassified from net accumulated other comprehensive loss		0.0	(0.2)	0.0	(0.2)
Net current period other comprehensive loss		0.0	0.0	0.0	0.0
Accumulated other comprehensive loss at end of period		(0.2)	(0.3)	(0.2)	(0.3)
Defined Benefit 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), tax		0.0	(0.1)	0.0	(0.1)
Interest rate derivatives, tax		0.0	0.0	0.0	0.0
Amortization of net actuarial loss, tax		0.0	0.1	0.0	0.1
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	[1]	0.0	0.0	0.0	0.0
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		(24.9)	(26.1)	(25.5)	(26.7)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		0.0	(0.2)	0.0	(0.2)
Losses reclassified from net accumulated other comprehensive loss		0.0	(0.2)	0.0	(0.2)
Net current period other comprehensive loss		0.3	0.3	0.9	0.9
Accumulated other comprehensive loss at end of period		(24.6)	(25.8)	(24.6)	(25.8)
Total | Interest Rate Swap
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Losses reclassified from net accumulated other comprehensive loss	[1]	$ (0.3)	$ (0.3)	$ (0.9)	$ (0.9)

[1] Included in interest charges.