Report of Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010	Aug. 31, 2012	Nov. 01, 2011
Segment Reporting Information [Line Items]
Purchase Price														$ 78.6	$ 200.4
Operating revenues	862.1	1,113.4	855.0	840.7	885.2	1,212.1	978.1	840.5	3,671.2		3,915.9		3,716.9
Cost of goods sold									1,918.7		2,277.9		2,187.4
Gross margin on revenues									1,752.5		1,638.0		1,529.5
Other operation and maintenance									601.5		581.2		549.8
Depreciation and amortization									371.0		307.1		291.3
Impairment of assets									0.4		6.3		1.1
Gain on insurance proceeds									(7.5)		(3.0)		0
Taxes other than income									110.2		99.7		93.4
Operating income (loss)	97.3	304.0	177.3	98.3	96.9	299.7	182.2	67.9	676.9		646.7		593.9
Total assets	9,922.2				8,906.0				9,922.2		8,906.0		7,669.1
Capital expenditures									1,229.2	[1]	1,470.8	[2]	879.9
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues									2,141.2		2,211.5		2,109.9
Cost of goods sold									879.1		1,013.5		1,000.2
Gross margin on revenues									1,262.1		1,198.0		1,109.7
Other operation and maintenance									446.3		436.0		418.1
Depreciation and amortization									248.7		216.1		208.7
Impairment of assets									0		0		0
Gain on insurance proceeds									0		0
Taxes other than income									77.7		73.6		69.2
Operating income (loss)									489.4		472.3		413.7
Total assets	7,222.4				6,620.9				7,222.4		6,620.9		5,898.1
Capital expenditures									704.4		844.5		631.6
Transportation and Storage [Member]
Segment Reporting Information [Line Items]
Operating revenues									639.5		880.1		984.8
Cost of goods sold									504.9		736.0		834.5
Gross margin on revenues									134.6		144.1		150.3
Other operation and maintenance									49.8		50.7		53.8
Depreciation and amortization									24.0		22.0		21.2
Impairment of assets									0		0		0.7
Gain on insurance proceeds									0		0
Taxes other than income									15.7		15.0		14.2
Operating income (loss)									45.1		56.4		60.4
Total assets	2,330.8				1,836.9				2,330.8		1,836.9		1,316.6
Capital expenditures									32.0	[1]	41.1	[2]	72.6
Gathering and Processing [Member]
Segment Reporting Information [Line Items]
Operating revenues									1,222.6		1,167.1		1,005.6
Cost of goods sold									868.7		870.7		733.3
Gross margin on revenues									353.9		296.4		272.3
Other operation and maintenance									123.1		111.8		91.5
Depreciation and amortization									84.8		55.6		50.1
Impairment of assets									0.4		6.3		0.4
Gain on insurance proceeds									(7.5)		(3.0)
Taxes other than income									12.6		7.0		6.4
Operating income (loss)									140.5		118.7		123.9
Total assets	1,868.6				1,483.8				1,868.6		1,483.8		973.8
Capital expenditures									475.4	[1]	572.0	[2]	164.0
Other Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues									0		0		0
Cost of goods sold									0		0		0
Gross margin on revenues									0		0		0
Other operation and maintenance									(17.7)		(17.3)		(13.6)
Depreciation and amortization									13.5		13.4		11.3
Impairment of assets									0		0		0
Gain on insurance proceeds									0		0
Taxes other than income									4.2		4.1		3.6
Operating income (loss)									0		(0.2)		(1.3)
Total assets	272.6				166.6				272.6		166.6		135.4
Capital expenditures									18.3	[1]	13.8	[2]	14.1
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues									(332.1)		(342.8)		(383.4)
Cost of goods sold									(334.0)		(342.3)		(380.6)
Gross margin on revenues									1.9		(0.5)		(2.8)
Other operation and maintenance									0		0		0
Depreciation and amortization									0		0		0
Impairment of assets									0		0		0
Gain on insurance proceeds									0		0
Taxes other than income									0		0		0
Operating income (loss)									1.9		(0.5)		(2.8)
Total assets	(1,772.2)				(1,202.2)				(1,772.2)		(1,202.2)		(654.8)
Capital expenditures									$ (0.9)	[1]	$ (0.6)	[2]	$ (2.4)

[1] Includes $78.6 million related to the acquisition of certain gas gathering assets as discussed in Note 3.

[2] Includes $200.4 million related to the acquisition of certain gas gathering assets as discussed in Note 3.