Fair Value of Financial Assets and Liabilities Consideration of Credit Risk and Concentrations (Details)	3 Months Ended
	Mar. 31, 2020 USD ($) Counterparty		Mar. 31, 2019 USD ($)		Dec. 31, 2019 USD ($)
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Derivative instruments designated as fair value hedges	$ 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
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities			3,300,000
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	0		0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	(3,400,000)		(1,300,000)
Pre-tax gains (losses) recognized during the period in income	(5,500,000)		(1,100,000)
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
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	0		0
Pre-tax gains (losses) recognized during the period in income	(2,100,000)	[1]	900,000	[2]
Not Designated as Hedging Instrument [Member] | Natural Gas 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
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	0		3,300,000
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	0		0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	(3,400,000)	[3]	(1,300,000)	[4]
Pre-tax gains (losses) recognized during the period in income	(3,400,000)	[3]	(2,000,000.0)	[4]
Not Designated as Hedging Instrument [Member] | Natural Gas Commodity for Electric Generation [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
Designated as Hedging Instrument [Member] | Cash Flow Hedging [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	400,000		400,000
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	0		0
Pre-tax gains (losses) recognized during the period in income	0		0
Designated as Hedging Instrument [Member] | Interest Rate Contract [Member] | Cash Flow Hedging [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	400,000	[5]	400,000	[6]
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	0		0
Pre-tax gains (losses) recognized during the period in income	$ 0		$ 0
Credit Concentration Risk [Member]
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty	10
Credit Concentration Risk [Member] | 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	8
Credit Concentration Risk [Member] | 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	4
Wholesale, trading and non trading commodity credit exposure for the most significant counterparties	$ 113,300,000
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties	77.00%
Credit Concentration Risk [Member] | Internal Investment Grade [Member]
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty	5
Wholesale, trading and non trading commodity credit exposure for the most significant counterparties	$ 20,600,000
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties	14.00%
Credit Concentration Risk [Member] | 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
Wholesale, trading and non trading commodity credit exposure for the most significant counterparties	$ 2,300,000
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties	1.00%

[1] 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.

[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 for the three months ended March 31, 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 months ended March 31, 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.

[4] Amounts for the three months ended March 31, 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 months ended March 31, 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.

[5] Amounts are recorded to interest charges.

[6] Amounts are recorded to interest charges.