Business Segment Information (Schedule Of Operating Income Derived From Revenues And Expenses By Segment) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2015		Jun. 30, 2014		Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information [Line Items]
Revenues	$ 1,169.0		$ 1,335.1		$ 3,318.7	$ 3,655.6
Consolidated Operating Income (Loss)	165.1		219.6		695.2	753.3
Gas Distribution Operations [Member]
Segment Reporting Information [Line Items]
Revenues	508.7		616.6		1,965.0	2,182.2
Consolidated Operating Income (Loss)	49.7		59.8		374.9	361.6
Gas Distribution Operations [Member] | Unaffiliated [Member]
Segment Reporting Information [Line Items]
Revenues	508.5		616.5		1,964.7	2,181.9
Gas Distribution Operations [Member] | Intersegment [Member]
Segment Reporting Information [Line Items]
Revenues	0.2		0.1		0.3	0.3
Electric Operations [Member]
Segment Reporting Information [Line Items]
Revenues	375.7		405.4		771.5	855.8
Consolidated Operating Income (Loss)	45.7		62.9		115.7	141.8
Electric Operations [Member] | Unaffiliated [Member]
Segment Reporting Information [Line Items]
Revenues	375.5		405.3		771.1	855.5
Electric Operations [Member] | Intersegment [Member]
Segment Reporting Information [Line Items]
Revenues	0.2		0.1		0.4	0.3
Columbia Pipeline Group Operations [Member]
Segment Reporting Information [Line Items]
Revenues	315.9		343.5		655.7	689.1
Consolidated Operating Income (Loss)	108.6		103.7		271.6	262.6
Columbia Pipeline Group Operations [Member] | Unaffiliated [Member]
Segment Reporting Information [Line Items]
Revenues	284.3		311.3		581.7	614.5
Columbia Pipeline Group Operations [Member] | Intersegment [Member]
Segment Reporting Information [Line Items]
Revenues	31.6		32.2		74.0	74.6
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenues	120.2		130.9		250.6	259.4
Consolidated Operating Income (Loss)	(38.9)	[1]	(6.8)	[1]	(67.0)	(12.7)
Corporate and Other [Member] | Unaffiliated [Member]
Segment Reporting Information [Line Items]
Revenues	0.7		2.0		1.2	3.7
Corporate and Other [Member] | Intersegment [Member]
Segment Reporting Information [Line Items]
Revenues	119.5		128.9		249.4	255.7
Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	$ (151.5)		$ (161.3)		$ (324.1)	$ (330.9)

[1] Primarily resulting from costs associated with the Separation.