Other Commitments And Contingencies (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Other Commitments And Contingencies [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 8,452.2
Revolving credit facility, maximum	2,000.0
Line of Credit Facility, Expiration Date	Sep. 28, 2018
Line of Credit Facility, Amount Outstanding	0us-gaap_LineOfCredit	500.0us-gaap_LineOfCredit
Additional purchase and sales agreement guarantees	25.6
Recorded reserves to cover environmental remediation at various sites	128.3	128.4
Transmission Upgrade Agreement Multiplier	1.71
MGP Sites
Other Commitments And Contingencies [Line Items]
Number of waste disposal sites identified by program	66
Liability for Estimated Remediation Costs	120.3	121.5
Reasonably possible remediation costs variance from reserve	25
Columbia Pipeline Partners LP [Member]
Other Commitments And Contingencies [Line Items]
Revolving credit facility, maximum	500.0
Line of Credit Facility, Remaining Borrowing Capacity	50.0
Letters of Credit Outstanding, Amount	0
Millennium Pipeline Company, L.L.C [Member]
Other Commitments And Contingencies [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	16.2
Percentage of interest in Millennium	47.50%
Fair value of guarantee obligations	1.5
Guarantees Of Subsidiaries Debt [Member]
Other Commitments And Contingencies [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	7,960.5
Letters Of Credit [Member]
Other Commitments And Contingencies [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	30.9
Other Guarantees [Member]
Other Commitments And Contingencies [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	146.8
Lines Of Credit [Member]
Other Commitments And Contingencies [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	39.0
Accounts Receivable Securitization [Member]
Other Commitments And Contingencies [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	275.0
Maximum [Member] \| NIPSCO [Member]
Other Commitments And Contingencies [Line Items]
Remaining Payments for Capital Improvements	107.3
Payments for Capital Improvements	870
Letters Of Credit [Member]
Other Commitments And Contingencies [Line Items]
Letters of Credit Outstanding, Amount	30.9	30.9
Revolving Credit Facility [Member]
Other Commitments And Contingencies [Line Items]
Line of Credit Facility, Amount Outstanding	0
Commercial Paper [Member]
Other Commitments And Contingencies [Line Items]
Revolving credit facility, maximum	$ 1,500.0

Guarantee Obligations Fair Value.

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- Definition

Liability for Estimated Remediation Costs

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- Definition

Number of waste disposal sites identified by program.

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- Definition

Purchase sale agreements additional guarantee obligations.

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No definition available.

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- Definition

No authoritative reference available.

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No definition available.

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- Definition

Remaining Payments for Capital Improvements

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- Definition

Transmission Upgrade Agreement Multiplier

+ References

No definition available.

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- Definition

Total costs accrued as of the balance sheet date for environmental loss contingencies.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 30

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- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 460

-SubTopic 10

-Section 50

-Paragraph 4

-Subparagraph (b)(1)

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- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

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-Subparagraph (SX 210.7-03.16)

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- Definition

Date the credit facility terminates, in CCYY-MM-DD format.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

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- Definition

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

+ References

No definition available.

+ Details

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- Definition

The cash outflow for acquisition of or capital improvements to properties held for investment (operating, managed, leased) or for use.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Glossary Investing Activities

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-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 13

-Subparagraph (c)

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