Report of Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment Reporting Information [Line Items]
Operating revenues	$ 855.0		$ 978.1	$ 1,695.7	$ 1,818.6
Cost of goods sold	409.3		550.1	894.6	1,083.3
Gross margin on revenues	445.7		428.0	801.1	735.3
Other operation and maintenance	153.0		146.6	300.6	284.9
Depreciation and amortization	90.5		74.7	177.1	148.7
Impairment of assets	0.1		0	0.3	0
Gain on insurance proceeds	0	(3.0)	0	(7.5)	0
Taxes other than income	24.8		24.5	55.0	51.6
Operating income (loss)	177.3		182.2	275.6	250.1
Total assets	9,351.4	8,906.0	8,135.5	9,351.4	8,135.5
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	528.0		568.7	954.7	990.8
Cost of goods sold	204.6		254.3	400.1	473.7
Gross margin on revenues	323.4		314.4	554.6	517.1
Other operation and maintenance	114.7		110.2	225.3	216.0
Depreciation and amortization	62.7		52.1	122.4	103.9
Impairment of assets	0			0
Gain on insurance proceeds				0
Taxes other than income	18.2		18.8	39.3	37.9
Operating income (loss)	127.8		133.3	167.6	159.3
Total assets	6,833.2		6,290.5	6,833.2	6,290.5
Transportation and Storage [Member]
Segment Reporting Information [Line Items]
Operating revenues	75.1		108.0	154.2	208.2
Cost of goods sold	37.4		69.5	80.9	133.5
Gross margin on revenues	37.7		38.5	73.3	74.7
Other operation and maintenance	10.6		13.0	21.4	22.1
Depreciation and amortization	5.4		5.8	10.6	11.2
Impairment of assets	0			0
Gain on insurance proceeds				0
Taxes other than income	3.5		3.3	8.2	7.6
Operating income (loss)	18.2		16.4	33.1	33.8
Total assets	2,051.8		1,474.8	2,051.8	1,474.8
Gathering and Processing [Member]
Segment Reporting Information [Line Items]
Operating revenues	266.4		289.1	570.9	555.8
Cost of goods sold	178.6		210.9	396.5	407.2
Gross margin on revenues	87.8		78.2	174.4	148.6
Other operation and maintenance	29.9		26.3	60.0	53.1
Depreciation and amortization	18.6		13.4	36.4	26.9
Impairment of assets	0.1			0.3
Gain on insurance proceeds				(7.5)
Taxes other than income	2.1		1.7	4.6	3.6
Operating income (loss)	37.1		36.8	80.6	65.0
Total assets	1,623.0		1,106.7	1,623.0	1,106.7
Marketing [Member]
Segment Reporting Information [Line Items]
Operating revenues	76.6		168.3	206.6	366.4
Cost of goods sold	79.5		170.4	206.7	369.7
Gross margin on revenues	(2.9)		(2.1)	(0.1)	(3.3)
Other operation and maintenance	2.2		2.0	4.4	4.1
Depreciation and amortization	0.3		0.1	0.7	0.1
Impairment of assets	0			0
Gain on insurance proceeds				0
Taxes other than income	0.1		(0.1)	0.2	0.1
Operating income (loss)	(5.5)		(4.1)	(5.4)	(7.6)
Total assets	47.5		80.2	47.5	80.2
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)		(4.2)	(9.7)	(8.9)
Depreciation and amortization	3.5		3.3	7.0	6.6
Impairment of assets	0			0
Gain on insurance proceeds				0
Taxes other than income	0.9		0.8	2.7	2.4
Operating income (loss)	0		0.1	0	(0.1)
Total assets	331.4		152.4	331.4	152.4
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	(91.1)		(156.0)	(190.7)	(302.6)
Cost of goods sold	(90.8)		(155.0)	(189.6)	(300.8)
Gross margin on revenues	(0.3)		(1.0)	(1.1)	(1.8)
Other operation and maintenance	0		(0.7)	(0.8)	(1.5)
Depreciation and amortization	0		0	0	0
Impairment of assets	0			0
Gain on insurance proceeds				0
Taxes other than income	0		0	0	0
Operating income (loss)	(0.3)		(0.3)	(0.3)	(0.3)
Total assets	$ (1,535.5)		$ (969.1)	$ (1,535.5)	$ (969.1)