Discontinued Operations (Income Statement Disclosures) (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2015		Sep. 30, 2014		Sep. 30, 2015		Sep. 30, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Group, Including Discontinued Operation, Natural Gas Midstream Revenue	$ 0.0		$ 243.1		$ 561.4		$ 746.1
Disposal Group, Including Discontinued Operation, Other Revenue, Net	0.0		74.5		94.3		260.6
Disposal Group, Including Discontinued Operation, Revenue	0.0		317.6		655.7		1,006.7
Disposal Group, Including Discontinued Operation, Costs of Goods Sold	0.0		0.0		0.2		0.2
Disposal Group, Including Discontinued Operation, Gross Profit (Loss)	0.0		317.6		655.5		1,006.5
Disposal Group, Including Discontinued Operation, Utilities Operating Expense, Maintenance and Operations	5.5		202.1		374.8		573.3
Disposal Group, Including Discontinued Operation, Depreciation and Amortization	0.0		29.2		66.4		87.7
Disposal Group, Including Discontinued Operation, Impairment And Gain Loss On Sale Of Assets, Net	0.0		(3.0)		(13.6)		(20.8)
Disposal Group, Including Discontinued Operation, Taxes, Miscellaneous	0.0		14.6		38.0		50.3
Disposal Group, Including Discontinued Operation, Operating Expense	5.5		242.9		465.6		690.5
Disposal Group, Including Discontinued Operation, Income Loss From Equity Method Investments	0.0		12.0		29.1		32.9
Disposal Group, Including Discontinued Operation, Operating Income (Loss)	(5.5)		86.7		219.0		348.9
Disposal Group, Including Discontinued Operation, Interest Expense	0.0		(14.9)		(37.1)		(40.4)
Disposal Group, Including Discontinued Operation, Other Expense	0.0		3.3		7.2		6.8
Disposal Group, Including Discontinued Operation, Nonoperating Income Expense	0.0		(11.6)		(29.9)		(33.6)
Disposal Group, Including Discontinued Operation, Income Loss From Discontinued Operation Before Income Taxes Domestic	(5.5)		75.1		189.1		315.3
Disposal Group, Including Discontinued Operation, Income Tax Expense Benefit	14.2		26.5		80.6		116.2
(Loss) Income from Discontinued Operations - net of taxes	(19.7)		48.6		108.5		199.1
Columbia Pipeline Group Operations [Member] | Discontinued Operations, Disposed of by Means Other than Sale, Spinoff [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Group, Including Discontinued Operation, Natural Gas Midstream Revenue	0.0		243.1		561.4		746.1
Disposal Group, Including Discontinued Operation, Other Revenue, Net	0.0		74.5		94.3		260.6
Disposal Group, Including Discontinued Operation, Revenue	0.0		317.6		655.7		1,006.7
Disposal Group, Including Discontinued Operation, Costs of Goods Sold	0.0		0.0		0.2		0.2
Disposal Group, Including Discontinued Operation, Gross Profit (Loss)	0.0		317.6		655.5		1,006.5
Disposal Group, Including Discontinued Operation, Utilities Operating Expense, Maintenance and Operations	5.5	[1]	202.1	[2]	374.8	[1]	573.3	[2]
Disposal Group, Including Discontinued Operation, Depreciation and Amortization	0.0		29.2		66.4		87.7
Disposal Group, Including Discontinued Operation, Impairment And Gain Loss On Sale Of Assets, Net	0.0		(3.0)		(13.6)		(20.8)
Disposal Group, Including Discontinued Operation, Taxes, Miscellaneous	0.0		14.6		38.0		50.3
Disposal Group, Including Discontinued Operation, Operating Expense	5.5		242.9		465.6		690.5
Disposal Group, Including Discontinued Operation, Income Loss From Equity Method Investments	0.0		12.0		29.1		32.9
Disposal Group, Including Discontinued Operation, Operating Income (Loss)	(5.5)		86.7		219.0		348.9
Disposal Group, Including Discontinued Operation, Interest Expense	0.0		(14.9)		(37.1)		(40.4)
Disposal Group, Including Discontinued Operation, Other Expense	0.0		3.5		7.8		7.8
Disposal Group, Including Discontinued Operation, Nonoperating Income Expense	0.0		(11.4)		(29.3)		(32.6)
Disposal Group, Including Discontinued Operation, Income Loss From Discontinued Operation Before Income Taxes Domestic	(5.5)		75.3		189.7		316.3
Disposal Group, Including Discontinued Operation, Income Tax Expense Benefit	14.2	[3]	26.6		80.9		116.6
(Loss) Income from Discontinued Operations - net of taxes	(19.7)		48.7		108.8		199.7
Corporate and Other [Member] | Discontinued Operations, Disposed of by Means Other than Sale, Spinoff [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Group, Including Discontinued Operation, Natural Gas Midstream Revenue	0.0		0.0		0.0		0.0
Disposal Group, Including Discontinued Operation, Other Revenue, Net	0.0		0.0		0.0		0.0
Disposal Group, Including Discontinued Operation, Revenue	0.0		0.0		0.0		0.0
Disposal Group, Including Discontinued Operation, Costs of Goods Sold	0.0		0.0		0.0		0.0
Disposal Group, Including Discontinued Operation, Gross Profit (Loss)	0.0		0.0		0.0		0.0
Disposal Group, Including Discontinued Operation, Utilities Operating Expense, Maintenance and Operations	0.0		0.0		0.0		0.0
Disposal Group, Including Discontinued Operation, Depreciation and Amortization	0.0		0.0		0.0		0.0
Disposal Group, Including Discontinued Operation, Impairment And Gain Loss On Sale Of Assets, Net	0.0		0.0		0.0		0.0
Disposal Group, Including Discontinued Operation, Taxes, Miscellaneous	0.0		0.0		0.0		0.0
Disposal Group, Including Discontinued Operation, Operating Expense	0.0		0.0		0.0		0.0
Disposal Group, Including Discontinued Operation, Income Loss From Equity Method Investments	0.0		0.0		0.0		0.0
Disposal Group, Including Discontinued Operation, Operating Income (Loss)	0.0		0.0		0.0		0.0
Disposal Group, Including Discontinued Operation, Interest Expense	0.0		0.0		0.0		0.0
Disposal Group, Including Discontinued Operation, Other Expense	0.0		(0.2)		(0.6)		(1.0)
Disposal Group, Including Discontinued Operation, Nonoperating Income Expense	0.0		(0.2)		(0.6)		(1.0)
Disposal Group, Including Discontinued Operation, Income Loss From Discontinued Operation Before Income Taxes Domestic	0.0		(0.2)		(0.6)		(1.0)
Disposal Group, Including Discontinued Operation, Income Tax Expense Benefit	0.0		(0.1)		(0.3)		(0.4)
(Loss) Income from Discontinued Operations - net of taxes	$ 0.0		$ (0.1)		$ (0.3)		$ (0.6)

[1] Includes approximately $5.5 million and $54.4 million for the three and nine months ended September 30, 2015, of transaction costs related to the Separation.

[2] Includes approximately $9.3 million and $12.8 million for the three and nine months ended September 30, 2014, of transaction costs related to the Separation.

[3] Primarily attributable to the write-off of consolidated state income tax benefits resulting from the Separation.