Report of Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Operating revenues	$ 734.2	$ 855.0	$ 1,635.6	$ 1,695.7
Cost of goods sold	370.6	409.3	923.6	894.6
Gross margin on revenues	363.6	445.7	712.0	801.1
Other operation and maintenance	122.0	153.0	270.0	300.6
Depreciation and amortization	74.4	90.5	166.3	177.1
Impairment of assets	0	0.1	0	0.3
Gain on insurance proceeds	0	0	0	(7.5)
Taxes other than income	23.3	24.8	56.4	55.0
Operating income (loss)	143.9	177.3	219.3	275.6
Equity in earnings of unconsolidated affiliates	18.5	0	18.5	0
Investment in unconsolidated affiliates	1,267.2		1,267.2		0
Total assets	8,848.5	9,351.4	8,848.5	9,351.4	9,922.2
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	574.6	528.0	1,030.1	954.7
Cost of goods sold	247.6	204.6	460.6	400.1
Gross margin on revenues	327.0	323.4	569.5	554.6
Other operation and maintenance	107.0	114.7	212.1	225.3
Depreciation and amortization	62.0	62.7	123.3	122.4
Impairment of assets		0		0
Gain on insurance proceeds				0
Taxes other than income	19.9	18.2	43.1	39.3
Operating income (loss)	138.1	127.8	191.0	167.6
Equity in earnings of unconsolidated affiliates	0		0
Investment in unconsolidated affiliates	0		0
Total assets	7,453.6	6,833.2	7,453.6	6,833.2
Natural Gas Midstream Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	166.1	344.0	630.4	773.6
Cost of goods sold	129.7	222.6	489.0	527.9
Gross margin on revenues	36.4	121.4	141.4	245.7
Other operation and maintenance	15.7	42.7	60.9	85.0
Depreciation and amortization	9.2	24.3	36.8	47.7
Impairment of assets		0.1		0.3
Gain on insurance proceeds				(7.5)
Taxes other than income	2.5	5.7	10.5	13.0
Operating income (loss)	9.0	48.6	33.2	107.2
Equity in earnings of unconsolidated affiliates	18.5		18.5
Investment in unconsolidated affiliates	1,267.2		1,267.2
Total assets	1,266.8	2,395.3	1,266.8	2,395.3
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	(0.7)	(4.4)	(3.0)	(9.7)
Depreciation and amortization	3.2	3.5	6.2	7.0
Impairment of assets		0		0
Gain on insurance proceeds				0
Taxes other than income	0.9	0.9	2.8	2.7
Operating income (loss)	(3.4)	0	(6.0)	0
Equity in earnings of unconsolidated affiliates	0		0
Investment in unconsolidated affiliates	0		0
Total assets	189.4	331.4	189.4	331.4
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	(6.5)	(17.0)	(24.9)	(32.6)
Cost of goods sold	(6.7)	(17.9)	(26.0)	(33.4)
Gross margin on revenues	0.2	0.9	1.1	0.8
Other operation and maintenance	0	0	0	0
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)	0.2	0.9	1.1	0.8
Equity in earnings of unconsolidated affiliates	0		0
Investment in unconsolidated affiliates	0		0
Total assets	$ (61.3)	$ (208.5)	$ (61.3)	$ (208.5)