Qualifying Cash Flow Hedges (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financial Impact of Qualifying Cash Flow Hedges on Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated other comprehensive loss related to cash flow hedges at Jan. 1	$ 0.0	$ (0.1)	$ (0.1)
After-tax net realized losses on derivative transactions reclassified into earnings	0.0	0.1	0.0
Accumulated other comprehensive loss related to cash flow hedges at Dec. 31	0.0	0.0	(0.1)
Fair Value Hedges, Net	0.0	0.0	0.0
Not Designated as Hedging Instrument \| Natural Gas Commodity Contract
Financial Impact of Qualifying Cash Flow Hedges on Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Derivative Instruments Gain (Loss) Reclassified To Regulatory Assets And Liabilities Net	(0.8)	(0.1)	(0.3)
Pre-tax gains (losses) reclassified into income during the period from regulatory assets and (liabilities)	0.8	0.3	0.2
Cash Flow Hedges \| Designated as Hedging Instrument \| Interest Rate Contract
Financial Impact of Qualifying Cash Flow Hedges on Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	$ 0.0	$ 0.1	$ 0.1

Derivative Instruments Gain (Loss) Reclassified To Regulatory Assets And Liabilities Net

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Pre-tax gains (losses) reclassified into income during the period from regulatory assets and (liabilities)

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- Definition

Accumulated change, net of tax, in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of cash flow hedges. Includes an entity's share of an equity investee's Increase or Decrease in deferred hedging gains or losses.

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- Definition

The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

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- Definition

Net fair value of all derivative instruments designated as fair value hedging instruments.

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- Definition

Amount after tax of reclassification adjustment from accumulated other comprehensive income of accumulated gain (loss) realized from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's deferred hedging gain (loss).

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