Fair Value of Financial Assets and Liabilities Consideration of Credit Risk and Concentrations (Details) $ in Millions		3 Months Ended
		Mar. 31, 2020 USD ($) Counterparty	Mar. 31, 2019 USD ($)
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Fair Value Hedges, Net		$ 0.0	$ 0.0
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
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities			6.2
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)		(0.1)	(0.9)
Pre-tax gains (losses) recognized during the period in income		1.5	(0.8)
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
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		0.0	0.9
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)	[1]	(1.1)	(0.2)
Pre-tax gains (losses) recognized during the period in income	[1]	(2.0)	(1.3)
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
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		0.0	5.3
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)	[2]	(1.2)	(0.7)
Pre-tax gains (losses) recognized during the period in income		0.0	0.0
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	[3]	3.5	0.5
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		0.3	0.3
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] | 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	[4]	0.3	0.3
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
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty		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		8
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		7
Wholesale, trading and non trading commodity credit exposure for the most significant counterparties		$ 49.0
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties		55.00%
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		2
Wholesale, trading and non trading commodity credit exposure for the most significant counterparties		$ 15.5
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties		17.00%
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
Wholesale, trading and non trading commodity credit exposure for the most significant counterparties		$ 12.2
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties		14.00%

[1] 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.

[2] 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.

[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 recorded to interest charges.