Consideration of Credit Risk and Concentrations (Details) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2020 USD ($) Counterparty		Sep. 30, 2019 USD ($)		Sep. 30, 2020 USD ($) Counterparty		Sep. 30, 2019 USD ($)
Consideration of Credit Risk and Concentrations [Abstract]
Derivative instruments designated as fair value hedges	$ 0.0		$ 0.0		$ 0.0		$ 0.0
Other Derivative Instruments
Consideration of Credit Risk and Concentrations [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	0.6		(2.2)		(2.2)		(3.7)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	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	3.2		0.6		(5.5)		3.2
Other Derivative Instruments | Natural Gas Commodity
Consideration of Credit Risk and Concentrations [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	0.6		(2.2)		(2.2)		(3.7)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net					0.0		0.0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)					(3.4)	[1]	(1.3)	[2]
Pre-tax gains (losses) recognized during the period in income					(3.4)	[1]	(2.1)	[2]
Other Derivative Instruments | Commodity Trading
Consideration of Credit Risk and Concentrations [Abstract]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	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	3.2	[3]	0.6	[4]	(2.1)	[3]	5.3	[4]
Other Derivative Instruments | Natural Gas [Member]
Consideration of Credit Risk and Concentrations [Abstract]
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	0.0		0.0		0.0		0.0
Designated as Hedging Instrument | Cash Flow Hedges
Consideration of Credit Risk and Concentrations [Abstract]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	0.4		0.4		1.2		1.2
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
Designated as Hedging Instrument | Interest Rate | Cash Flow Hedges
Consideration of Credit Risk and Concentrations [Abstract]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	0.4	[5]	0.4	[6]	1.2	[5]	1.2	[6]
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
Consideration of Credit Risk and Concentrations [Abstract]
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]
Consideration of Credit Risk and Concentrations [Abstract]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty	8				8
Credit Concentration Risk | External Credit Rating, Investment Grade [Member]
Consideration of Credit Risk and Concentrations [Abstract]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty	5				5
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties	$ 108.3				$ 108.3
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths)	74.00%				74.00%
Credit Concentration Risk | No Investment Grade Ratings from External Credit Rating Agencies [Member]
Consideration of Credit Risk and Concentrations [Abstract]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty	4				4
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties	$ 22.6				$ 22.6
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths)	15.00%				15.00%
Credit Concentration Risk | External Credit Rating, Non Investment Grade [Member]
Consideration of Credit Risk and Concentrations [Abstract]
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	$ 2.9				$ 2.9
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths)	2.00%				2.00%

[1] Amounts for the three and nine months ended Sept. 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 nine months ended Sept. 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.

[2] Amounts for the three and nine months ended Sept. 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 nine months ended Sept. 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.

[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 are recorded to interest charges.

[6] Amounts are recorded to interest charges.