Fair Value of Financial Assets and Liabilities - Impact of Derivative Activity (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2020		Jun. 30, 2019		Jun. 30, 2020		Jun. 30, 2019
Financial Impact of Qualifying Fair Value Hedges on Earnings [Abstract]
Derivative instruments designated as fair value hedges	$ 0.0		$ 0.0		$ 0.0		$ 0.0
Designated as Hedging Instrument | Cash Flow Hedges
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	0.4		0.4		0.8		0.8
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	0.0		0.0		0.0		0.0
Pre-tax gains (losses) recognized during the period in income	0.0		0.0		0.0		0.0
Designated as Hedging Instrument | Cash Flow Hedges | Interest Rate
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	0.4	[1]	0.4	[2]	0.8	[1]	0.8	[2]
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	0.0		0.0		0.0		0.0
Pre-tax gains (losses) recognized during the period in income	0.0		0.0		0.0		0.0
Other Derivative Instruments
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss	0.0		0.0		0.0		0.0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	(2.8)		(1.5)		(2.8)		(1.5)
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	0.0		0.0		0.0		0.0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	0.0		0.0		3.4		1.3
Pre-tax gains (losses) recognized during the period in income	(1.4)		3.3		(8.7)		2.7
Other Derivative Instruments | Commodity Trading
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	0.0		0.0		0.0		0.0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	0.0		0.0		0.0		0.0
Pre-tax gains (losses) recognized during the period in income	(1.4)	[3]	3.3	[4]	(5.3)	[3]	4.7	[4]
Other Derivative Instruments | Natural Gas Commodity
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss	0.0		0.0		0.0		0.0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	(2.8)		(1.5)		(2.8)		(1.5)
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss					0.0		0.0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)					3.4	[5]	1.3	[6]
Pre-tax gains (losses) recognized during the period in income					(3.4)	[5]	(2.0)	[6]
Other Derivative Instruments | Natural Gas [Member]
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	$ 0.0		$ 0.0		$ 0.0		$ 0.0

[1] Amounts are recorded to interest charges.

[2] Amounts are recorded to interest charges.

[3] Amounts are recorded to electric operating revenues. Portions of these gains and losses are subject to sharing with electric customers through margin-sharing mechanisms and deducted from gross revenue as appropriate.

[4] Amounts are recorded to electric operating revenues. Portions of these gains and losses are subject to sharing with electric customers through margin-sharing mechanisms and deducted from gross revenue as appropriate.

[5] Amounts for the three and six months ended June 30, 2020, included no settlement gain or losses on derivatives entered to mitigate natural gas price risk for electric generation recorded to electric fuel and purchased power, subject to cost-recovery mechanisms and reclassified to a regulatory asset, as appropriate. Remaining derivative settlement losses for the three and six months ended June 30, 2020, relate to natural gas operations and are recorded to cost of natural gas sold and transported. These gains and losses are subject to cost-recovery mechanisms and reclassified out of income to a regulatory asset or liability, as appropriate.

[6] Amounts for the three and six months ended June 30, 2019, included no settlement gain or losses on derivatives entered to mitigate natural gas price risk for electric generation recorded to electric fuel and purchased power, subject to cost-recovery mechanisms and reclassified to a regulatory asset, as appropriate. Remaining derivative settlement losses for the three and six months ended June 30, 2019, relate to natural gas operations and are recorded to cost of natural gas sold and transported. These gains and losses are subject to cost-recovery mechanisms and reclassified out of income to a regulatory asset or liability, as appropriate.