Report of Business Segments (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Segment Reporting Information [Line Items]
Operating revenues	$ 1,212.1	$ 1,125.4	$ 3,030.7	$ 2,888.4
Cost of goods sold	658.5	612.6	1,741.8	1,689.2
Gross margin on revenues	553.6	512.8	1,288.9	1,199.2
Other operation and maintenance	147.4	142.4	432.3	401.0
Depreciation and amortization	77.1	73.7	225.8	215.2
Asset Impairment Charges	5.0	0	5.0	0
Taxes other than income	24.4	22.5	76.0	70.5
OPERATING INCOME	299.7	274.2	549.8	512.5
Total assets	8,378.8	7,527.4	8,378.8	7,527.4	7,669.1
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	774.8	723.0	1,765.6	1,679.8
Cost of goods sold	334.7	311.2	808.4	792.8
Gross margin on revenues	440.1	411.8	957.2	887.0
Other operation and maintenance	108.3	110.8	324.3	305.9
Depreciation and amortization	54.9	53.1	158.8	153.4
Asset Impairment Charges	0		0
Taxes other than income	18.2	16.9	56.1	51.8
OPERATING INCOME	258.7	231.0	418.0	375.9
Total assets	6,451.8	5,882.7	6,451.8	5,882.7
Transportation and Storage [Member]
Segment Reporting Information [Line Items]
Operating revenues	103.7	103.5	311.9	311.7
Cost of goods sold	58.6	64.8	192.1	191.9
Gross margin on revenues	45.1	38.7	119.8	119.8
Other operation and maintenance	13.7	11.6	35.8	35.2
Depreciation and amortization	5.2	5.2	16.4	16.0
Asset Impairment Charges	0		0
Taxes other than income	3.5	3.3	11.1	10.6
OPERATING INCOME	22.7	18.6	56.5	58.0
Total assets	2,474.1	1,670.0	2,474.1	1,670.0
Gathering and Processing [Member]
Segment Reporting Information [Line Items]
Operating revenues	304.9	243.1	860.7	726.4
Cost of goods sold	233.2	178.9	640.4	527.5
Gross margin on revenues	71.7	64.2	220.3	198.9
Other operation and maintenance	28.8	22.0	81.9	66.8
Depreciation and amortization	13.4	12.6	40.4	37.5
Asset Impairment Charges	5.0		5.0
Taxes other than income	1.8	1.4	5.3	4.9
OPERATING INCOME	22.7	28.2	87.7	89.7
Total assets	1,189.1	941.1	1,189.1	941.1
Marketing [Member]
Segment Reporting Information [Line Items]
Operating revenues	160.0	206.5	526.4	641.2
Cost of goods sold	163.1	207.6	532.8	644.8
Gross margin on revenues	(3.1)	(1.1)	(6.4)	(3.6)
Other operation and maintenance	1.8	1.8	5.9	6.6
Depreciation and amortization	0	0	0	0
Asset Impairment Charges	0		0
Taxes other than income	0.1	0.1	0.3	0.3
OPERATING INCOME	(5.0)	(3.0)	(12.6)	(10.5)
Total assets	67.7	105.4	67.7	105.4
Other Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Cost of goods sold	0	0	0	0
Gross margin on revenues	0	0	0	0
Other operation and maintenance	(4.4)	(3.2)	(13.3)	(10.8)
Depreciation and amortization	3.6	2.8	10.2	8.3
Asset Impairment Charges	0		0
Taxes other than income	0.8	0.8	3.2	2.9
OPERATING INCOME	0	(0.4)	(0.1)	(0.4)
Total assets	3,149.8	2,834.4	3,149.8	2,834.4
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	(131.3)	(150.7)	(433.9)	(470.7)
Cost of goods sold	(131.1)	(149.9)	(431.9)	(467.8)
Gross margin on revenues	(0.2)	(0.8)	(2.0)	(2.9)
Other operation and maintenance	(0.8)	(0.6)	(2.3)	(2.7)
Depreciation and amortization	0	0	0	0
Asset Impairment Charges	0		0
Taxes other than income	0	0	0	0
OPERATING INCOME	0.6	(0.2)	0.3	(0.2)
Total assets	$ (4,953.7)	$ (3,906.2)	$ (4,953.7)	$ (3,906.2)