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Regulatory Matters (Schedule of COVID-19 Regulatory Impact) (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Regulated Operations [Line Items]
COVID-19 related regulatory asset	$ 2,492.2	$ 1,930.5
Incremental O&M - COVID | Columbia Of Ohio
Regulated Operations [Line Items]
COVID-19 related regulatory asset	2.1	2.0
Incremental O&M and Bad Debt-COVID | Columbia Of Pennsylvania
Regulated Operations [Line Items]
COVID-19 related regulatory asset	5.2	5.4
Incremental Bad Debt - COVID | NIPSCO
Regulated Operations [Line Items]
COVID-19 related regulatory asset	2.2	9.2
Incremental Bad Debt - COVID | Columbia Of Virginia
Regulated Operations [Line Items]
COVID-19 related regulatory asset	1.5	0.0
Incremental Bad Debt - COVID | Columbia Of Maryland
Regulated Operations [Line Items]
COVID-19 related regulatory asset	$ 0.9	$ 0.7

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- Definition

Regulated Operations [Line Items]

+ References

No definition available.

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- Definition

The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

+ 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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