Report of Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Nov. 01, 2011
Segment Reporting Information [Line Items]
Operating revenues	$ 885.2	$ 1,212.1	$ 978.1	$ 840.5	$ 828.5	$ 1,125.4	$ 887.2	$ 875.8	$ 3,915.9	$ 3,716.9	$ 2,869.7
Cost of goods sold									2,277.9	2,187.4	1,557.7
Gross margin on revenues									1,638.0	1,529.5	1,312.0
Other operation and maintenance									581.2	549.8	466.8
Depreciation and amortization									307.1	291.3	262.6
Impairment of assets									6.3	1.1	3.1
Gain on insurance proceeds									(3.0)	0	0
Taxes other than income									99.7	93.4	87.6
OPERATING INCOME	96.9	299.7	182.2	67.9	81.4	274.2	151.5	86.8	646.7	593.9	491.9
Total assets	8,906.0				7,669.1				8,906.0	7,669.1	7,266.7
Capital expenditures									1,470.8	879.9	847.8
Business Combination, Consideration Transferred												200.4
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues									2,211.5	2,109.9	1,751.2
Cost of goods sold									1,013.5	1,000.2	796.3
Gross margin on revenues									1,198.0	1,109.7	954.9
Other operation and maintenance									436.0	418.1	348.0
Depreciation and amortization									216.1	208.7	187.4
Impairment of assets									0	0	0.3
Gain on insurance proceeds									0
Taxes other than income									73.6	69.2	65.1
OPERATING INCOME									472.3	413.7	354.1
Total assets	6,620.9				5,898.1				6,620.9	5,898.1	5,478.1
Capital expenditures									844.5	631.6	600.5
Transportation and Storage [Member]
Segment Reporting Information [Line Items]
Operating revenues									410.5	403.6	401.0
Cost of goods sold									253.3	246.4	239.9
Gross margin on revenues									157.2	157.2	161.1
Other operation and maintenance									46.5	48.9	40.9
Depreciation and amortization									21.6	21.1	20.4
Impairment of assets									0	0.7	0.9
Gain on insurance proceeds									0
Taxes other than income									14.7	13.9	13.2
OPERATING INCOME									74.4	72.6	85.7
Total assets	1,805.5				1,246.1				1,805.5	1,246.1	1,159.5
Capital expenditures									39.3	70.2	71.4
Gathering and Processing [Member]
Segment Reporting Information [Line Items]
Operating revenues									1,167.1	1,005.6	657.5
Cost of goods sold									870.7	733.3	458.8
Gross margin on revenues									296.4	272.3	198.7
Other operation and maintenance									111.8	91.5	87.2
Depreciation and amortization									55.6	50.1	43.9
Impairment of assets									6.3	0.4	1.9
Gain on insurance proceeds									(3.0)
Taxes other than income									7.0	6.4	5.5
OPERATING INCOME									118.7	123.9	60.2
Total assets	1,483.8				973.8				1,483.8	973.8	866.1
Capital expenditures									572.0	164.0	166.0
Marketing [Member]
Segment Reporting Information [Line Items]
Operating revenues									678.0	798.5	619.9
Cost of goods sold									688.1	804.7	617.7
Gross margin on revenues									(10.1)	(6.2)	2.2
Other operation and maintenance									7.3	8.4	9.2
Depreciation and amortization									0.4	0.1	0.1
Impairment of assets									0	0	0
Gain on insurance proceeds									0
Taxes other than income									0.3	0.3	0.4
OPERATING INCOME									(18.1)	(15.0)	(7.5)
Total assets	74.5				94.5				74.5	94.5	125.2
Capital expenditures									1.8	2.4	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									(17.3)	(13.6)	(13.9)
Depreciation and amortization									13.4	11.3	10.8
Impairment of assets									0	0	0
Gain on insurance proceeds									0
Taxes other than income									4.1	3.6	3.4
OPERATING INCOME									(0.2)	(1.3)	(0.3)
Total assets	166.6				135.4				166.6	135.4	137.3
Capital expenditures									13.8	14.1	10.2
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues									(551.2)	(600.7)	(559.9)
Cost of goods sold									(547.7)	(597.2)	(555.0)
Gross margin on revenues									(3.5)	(3.5)	(4.9)
Other operation and maintenance									(3.1)	(3.5)	(4.6)
Depreciation and amortization									0	0	0
Impairment of assets									0	0	0
Gain on insurance proceeds									0
Taxes other than income									0	0	0
OPERATING INCOME									(0.4)	0	(0.3)
Total assets	(1,245.3)				(678.8)				(1,245.3)	(678.8)	(499.5)
Capital expenditures									$ (0.6)	$ (2.4)	$ (0.3)