Report of Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		5 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Operating revenues	$ 723.2	$ 1,113.4		$ 2,358.8	$ 2,809.1
Cost of goods sold	273.0	539.6		1,196.6	1,434.2
Gross margin on revenues	450.2	573.8		1,162.2	1,374.9
Other operation and maintenance	102.2	147.1		372.2	447.7
Depreciation and amortization	65.4	93.0		231.7	270.1
Impairment of assets	0	0		0	0.3
Gain on insurance proceeds	0	0		0	(7.5)
Taxes other than income	21.7	29.7		78.1	84.7
Operating income (loss)	260.9	304.0		480.2	579.6
Equity in earnings of unconsolidated affiliates	46.0	0	64.5	64.5	0
Investment in unconsolidated affiliates	1,295.8		1,295.8	1,295.8		0
Total assets	9,144.2	9,706.0	9,144.2	9,144.2	9,706.0	9,922.2
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	723.2	721.0		1,753.3	1,675.7
Cost of goods sold	273.0	271.8		733.6	671.9
Gross margin on revenues	450.2	449.2		1,019.7	1,003.8
Other operation and maintenance	105.9	108.6		318.0	333.9
Depreciation and amortization	62.5	63.5		185.8	185.9
Impairment of assets					0
Gain on insurance proceeds					0
Taxes other than income	20.8	19.1		63.9	58.4
Operating income (loss)	261.0	258.0		452.0	425.6
Equity in earnings of unconsolidated affiliates	0			0
Investment in unconsolidated affiliates	0		0	0
Total assets	7,704.0	7,082.3	7,704.0	7,704.0	7,082.3
Natural Gas Midstream Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	412.4		630.4	1,186.0
Cost of goods sold	0	288.6		489.0	816.5
Gross margin on revenues	0	123.8		141.4	369.5
Other operation and maintenance	0	42.3		60.9	127.3
Depreciation and amortization	0	26.5		36.8	74.2
Impairment of assets					0.3
Gain on insurance proceeds					(7.5)
Taxes other than income	0	9.8		10.5	22.8
Operating income (loss)	0	45.2		33.2	152.4
Equity in earnings of unconsolidated affiliates	46.0			64.5
Investment in unconsolidated affiliates	1,295.8		1,295.8	1,295.8
Total assets	1,311.3	2,562.7	1,311.3	1,311.3	2,562.7
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	(3.7)	(3.8)		(6.7)	(13.5)
Depreciation and amortization	2.9	3.0		9.1	10.0
Impairment of assets					0
Gain on insurance proceeds					0
Taxes other than income	0.9	0.8		3.7	3.5
Operating income (loss)	(0.1)	0		(6.1)	0
Equity in earnings of unconsolidated affiliates	0			0
Investment in unconsolidated affiliates	0		0	0
Total assets	172.2	276.6	172.2	172.2	276.6
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	(20.0)		(24.9)	(52.6)
Cost of goods sold	0	(20.8)		(26.0)	(54.2)
Gross margin on revenues	0	0.8		1.1	1.6
Other operation and maintenance	0	0		0	0
Depreciation and amortization	0	0		0	0
Impairment of assets					0
Gain on insurance proceeds					0
Taxes other than income	0	0		0	0
Operating income (loss)	0	0.8		1.1	1.6
Equity in earnings of unconsolidated affiliates	0			0
Investment in unconsolidated affiliates	0		0	0
Total assets	$ (43.3)	$ (215.6)	$ (43.3)	$ (43.3)	$ (215.6)