Report of Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Operating revenues	$ 901.4	$ 840.7
Cost of goods sold	553.0	485.3
Gross margin on revenues	348.4	355.4
Other operation and maintenance	148.0	147.6
Depreciation and amortization	91.9	86.6
Impairment of assets	0	0.2
Gain on insurance proceeds	0	(7.5)
Taxes other than income	33.1	30.2
Operating income (loss)	75.4	98.3
Total assets	9,981.8	9,074.5	9,922.2
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	455.5	426.7
Cost of goods sold	213.0	195.5
Gross margin on revenues	242.5	231.2
Other operation and maintenance	105.1	110.6
Depreciation and amortization	61.3	59.7
Impairment of assets		0
Gain on insurance proceeds		0
Taxes other than income	23.2	21.1
Operating income (loss)	52.9	39.8
Total assets	7,138.4	6,632.8
Transportation and Storage [Member]
Segment Reporting Information [Line Items]
Operating revenues	216.4	169.5
Cost of goods sold	182.7	131.8
Gross margin on revenues	33.7	37.7
Other operation and maintenance	10.9	12.1
Depreciation and amortization	5.8	5.6
Impairment of assets		0
Gain on insurance proceeds		0
Taxes other than income	4.8	4.8
Operating income (loss)	12.2	15.2
Total assets	2,453.0	1,950.3
Gathering and Processing [Member]
Segment Reporting Information [Line Items]
Operating revenues	317.9	304.5
Cost of goods sold	246.4	217.9
Gross margin on revenues	71.5	86.6
Other operation and maintenance	34.3	30.1
Depreciation and amortization	21.8	17.8
Impairment of assets		0.2
Gain on insurance proceeds		(7.5)
Taxes other than income	3.2	2.5
Operating income (loss)	12.2	43.5
Total assets	1,948.9	1,574.1
Other Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0
Cost of goods sold	0	0
Gross margin on revenues	0	0
Other operation and maintenance	(2.3)	(5.3)
Depreciation and amortization	3.0	3.5
Impairment of assets		0
Gain on insurance proceeds		0
Taxes other than income	1.9	1.8
Operating income (loss)	(2.6)	0
Total assets	387.2	257.7
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	(88.4)	(60.0)
Cost of goods sold	(89.1)	(59.9)
Gross margin on revenues	0.7	(0.1)
Other operation and maintenance	0	0.1
Depreciation and amortization	0	0
Impairment of assets		0
Gain on insurance proceeds		0
Taxes other than income	0	0
Operating income (loss)	0.7	(0.2)
Total assets	$ (1,945.7)	$ (1,340.4)