Fair Value of Financial Assets and Liabilities Consideration of Credit Risk and Concentrations (Details) $ in Millions		3 Months Ended			6 Months Ended
		Jun. 30, 2020 USD ($) Counterparty		Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) Counterparty		Jun. 30, 2019 USD ($)
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Fair Value Hedges, Net		$ 0.0		$ 0.0	$ 0.0		$ 0.0
Not Designated as Hedging Instrument [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 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		2.0		15.4	2.6		(0.8)
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)		(0.7)		0.0	(0.7)		1.0
Pre-tax gains (losses) recognized during the period in income		(1.3)		1.6	(1.3)		(2.5)
Not Designated as Hedging Instrument [Member] | Electric Commodity Contract [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 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		2.3		15.6	2.4		(0.5)
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss		0.0			0.0		0.0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)		(0.7)	[1]		(1.9)	[1]	0.8	[2]
Pre-tax gains (losses) recognized during the period in income		0.0			0.0		0.0
Not Designated as Hedging Instrument [Member] | Natural Gas Commodity
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
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	[3]				(1.2)		(0.2)
Pre-tax gains (losses) recognized during the period in income	[3]				(2.0)		(1.3)
Not Designated as Hedging Instrument [Member] | Commodity Trading Contract [Member]
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	[2]	(1.3)		1.6	0.7		(1.2)
Not Designated as Hedging Instrument [Member] | 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		(0.3)		(0.2)	0.2		(0.3)
Designated as Hedging Instrument [Member] | 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		0.2		0.3	0.5		0.5
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 [Member] | Cash Flow Hedges | Interest Rate Contract [Member]
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		0.2	[4]	0.3	0.5	[4]	0.5	[4]
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
Credit Concentration Risk
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty		10			10
Credit Concentration Risk | Municipal or Cooperative Entities or Other Utilities [Member]
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty		7			7
Credit Concentration Risk | External Credit Rating, Investment Grade [Member]
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty		6			6
Wholesale, trading and non trading commodity credit exposure for the most significant counterparties		$ 40.5			$ 40.5
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties		51.00%			51.00%
Credit Concentration Risk | Internal Investment Grade [Member]
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty		3			3
Wholesale, trading and non trading commodity credit exposure for the most significant counterparties		$ 16.0			$ 16.0
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties		20.00%			20.00%
Credit Concentration Risk | External Credit Rating, Non Investment Grade [Member]
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty		1			1
Wholesale, trading and non trading commodity credit exposure for the most significant counterparties		$ 13.2			$ 13.2
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties		17.00%			17.00%

[1] Amounts are recorded to electric fuel and purchased power. These derivative settlement gains and losses are shared with electric customers through fuel and purchased energy cost-recovery mechanisms, and reclassified out of income as regulatory assets or liabilities, as appropriate.

[2] 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.

[3] Amounts are recorded to cost of natural gas sold and transported. These derivative settlement gains and losses are shared with natural gas customers through purchased natural gas cost-recovery mechanisms, and reclassified out of income as regulatory assets or liabilities, as appropriate.

[4] Amounts recorded to interest charges.