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Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions																								3 Months Ended	9 Months Ended		12 Months Ended
	Oct. 31, 2016	Oct. 27, 2016	Oct. 20, 2016	Sep. 28, 2016	Aug. 26, 2016	Jul. 27, 2016	Jun. 30, 2016	May 27, 2016	May 01, 2016	Apr. 29, 2016	Apr. 15, 2016	Mar. 18, 2016	Mar. 16, 2016	Mar. 11, 2016	Feb. 26, 2016	Feb. 19, 2016	Jan. 29, 2016	Dec. 31, 2015	Oct. 30, 2015	Oct. 01, 2015	Dec. 17, 2014	Nov. 12, 2013	Jul. 19, 2013	Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Aug. 31, 2016	Jun. 22, 2016	Apr. 20, 2016	Mar. 24, 2016
Regulatory Matters [Line Items]
Seven Year Plan of Eligible Investments, Length of Plan																							seven-year
New Seven Year plan of Eligible Investments, Length of Plan																		seven-year
Increase (Decrease) in Regulatory Assets and Liabilities																									$ 202.2	$ (259.6)
Columbia Of Ohio
Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount															$ 25.9
Requested Regulatory Deferral														$ 25.0
Approved Regulatory Deferral					$ 25.0																$ 15.0
Columbia Of Massachusetts | 2016 GSEP
Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Cumulative Amount									$ 9.0
Public Utilities, Requested Rate Increase (Decrease), Revised Cumulative Amount									$ 8.2
Public Utilities, Requested Rate Increase (Decrease), Amount																			$ 6.4
Columbia Of Massachusetts | CMA Off-peak RDAF
Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount													$ 3.4
Columbia Of Massachusetts | CMA Off-Peak Local Distribution Adjustment Factor (LDAF)
Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount																	$ 42.0
Columbia Of Maryland
Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount											$ 6.5
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount						$ 3.7
Columbia Of Virginia
Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount										$ 37.0
Columbia Of Kentucky
Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount								$ 25.4
Columbia Of Pennsylvania
Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount												$ 55.3
Capital Investment Program Amount																											$ 197.0
Capital Investment Program Amount year 2												220.0
Capital Investment Program Amount year 3												$ 265.0
NIPSCO
Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount							$ 45.5
Amount of public utility's requested rate increase (decrease) with regulatory agency, before riders																$ 72.5				$ 126.6
New Seven year Plan of Eligible Investments Under TDSIC, Amount of Investment																		$ 1,300.0									$ 1,300.0
New Seven year Plan of Eligible Investments Under TDSIC, Amount of Investment, Updated																															$ 1,250.0
Public Utilities, Approved Return on Equity, Percentage																						12.38%
Public Utilities, Requested Return on Equity, Percentage, Amended				10.32%			9.70%																		10.82%
Liability associated with Public Utilities, requested amendment to ROE																								$ 5.3	$ 5.3
Increase (Decrease) in Regulatory Assets and Liabilities																								$ 7.8
NIPSCO | TDSIC 3 Gas
Regulatory Matters [Line Items]
Regulatory Net Capital Expenditures Included In Filing																													$ 145.7
Seven Year Plan of Eligible Investments Under TDSIC																													$ 800.0
NIPSCO | TDSIC 5 Gas
Regulatory Matters [Line Items]
Regulatory Net Capital Expenditures Included In Filing																												$ 211.6
NIPSCO | TDSIC 5 Gas
Regulatory Matters [Line Items]
Seven Year Plan of Eligible Investments Under TDSIC																												$ 824.0
NIPSCO | ECR 27
Regulatory Matters [Line Items]
Regulatory Net Capital Expenditures Included In Filing																														$ 800.7
NIPSCO | ECR 28
Regulatory Matters [Line Items]
Regulatory Net Capital Expenditures Included In Filing																														$ 267.0
Subsequent Event | Columbia Of Massachusetts | 2017 GSEP
Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Cumulative Amount	$ 16.8
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 8.1
Subsequent Event | Columbia Of Kentucky
Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount			$ 13.4
Subsequent Event | Columbia Of Pennsylvania
Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 35.0

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The increase (decrease) during the reporting period in the value of assets that are created when regulatory agencies permits public utilities to defer costs (revenues) to the balance sheet. This element is a the increase (decrease) of regulatory assets and liabilities combined.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585

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