Fair Value of Financial Assets and Liabilities Consideration of Credit Risk and Concentrations (Details)		3 Months Ended		9 Months Ended
		Sep. 30, 2019 USD ($) Counterparty	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) Counterparty	Sep. 30, 2018 USD ($)
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	$ 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
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		(2,200,000)	(1,200,000)	(3,700,000)	(1,600,000)
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)		0	0	(1,300,000)	2,700,000
Pre-tax gains (losses) recognized during the period in income		600,000	2,000,000.0	3,200,000	1,100,000
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	0	0	1,200,000
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	0	0	0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		(2,200,000)	(1,200,000)	(3,700,000)	(1,600,000)
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)	[1]			(1,300,000)	2,700,000
Pre-tax gains (losses) recognized during the period in income	[1]			(2,100,000)	(1,600,000)
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	0	0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)		0	0	0	0
Pre-tax gains (losses) recognized during the period in income	[2]	600,000	2,000,000.0	5,300,000	2,700,000
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	1,200,000	1,200,000
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)		0	0	0	0
Pre-tax gains (losses) recognized during the period in income		0	0	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	[3]	400,000	400,000	1,200,000	1,200,000
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)		0	0	0	0
Pre-tax gains (losses) recognized during the period in income		$ 0	$ 0	$ 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		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		7		7
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		6		6
Wholesale, trading and non trading commodity credit exposure for the most significant counterparties		$ 124,600,000		$ 124,600,000
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties		74.00%		74.00%
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		3		3
Wholesale, trading and non trading commodity credit exposure for the most significant counterparties		$ 14,900,000		$ 14,900,000
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties		9.00%		9.00%
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		1
Wholesale, trading and non trading commodity credit exposure for the most significant counterparties		$ 8,000,000.0		$ 8,000,000.0
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties		5.00%		5.00%

[1] Amounts for both 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. Amounts for the three and nine months ended Sept. 30, 2018 included no such settlement gains or losses and $1.2 million of such settlement losses, respectively. Remaining derivative settlement losses for the three and nine months ended Sept. 30, 2019 and 2018 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 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 interest charges.