Other Comprehensive Income (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		$ 2,536.6
Accumulated other comprehensive loss at end of period		2,884.5	$ 2,536.6
Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		(0.7)	(0.8)
Other Comprehensive Income (Loss) before reclassifications		0.0
Amortization of net actuarial loss		0.0
Net current period other comprehensive income (loss)		0.0	0.1
Accumulated other comprehensive loss at end of period		(0.7)	(0.7)
Gains and Losses on Cash Flow Hedges | Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amortization of net actuarial loss	[1]		0.1
Defined Benefit Pension and Postretirement Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		(0.7)	(0.7)
Other Comprehensive Income (Loss) before reclassifications		(0.2)
Amortization of net actuarial loss	[2]	0.2
Net current period other comprehensive income (loss)		0.0	0.0
Accumulated other comprehensive loss at end of period		(0.7)	(0.7)
Defined Benefit Pension and Postretirement Items | Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amortization of net actuarial loss			0.0
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		(1.4)	(1.5)
Other Comprehensive Income (Loss) before reclassifications		(0.2)
Amortization of net actuarial loss		0.2
Net current period other comprehensive income (loss)		0.0	0.1
Accumulated other comprehensive loss at end of period		$ (1.4)	(1.4)
Total | Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amortization of net actuarial loss			$ 0.1

[1] Included in interest charges.

[2] Included in the computation of net periodic pension and postretirement benefit costs. See Note 9 for further information.