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Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Regulatory Matters [Line Items]
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	$ 1,260.6	$ 1,524.3
Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	41 years
Columbia Of Ohio
Regulatory Matters [Line Items]
COVID-19 related costs regulatory asset	$ 2.0
NIPSCO
Regulatory Matters [Line Items]
COVID-19 related costs regulatory asset	9.2
Columbia Of Maryland
Regulatory Matters [Line Items]
COVID-19 related costs regulatory asset	0.7
Columbia Of Pennsylvania
Regulatory Matters [Line Items]
COVID-19 related costs regulatory asset	$ 5.4

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Regulatory Matters [Line Items]

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Remaining balance of regulatory assets for which no return on investment during recovery is provided. In some cases, a regulator may permit an enterprise to include a cost that would be charged to expense by an unregulated enterprise as an allowable cost over a period of time by amortizing that cost for rate-making purposes, but the regulator does not include the unrecovered amount in the rate base.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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-Topic 980
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-Section 55
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Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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-Topic 980
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