Report of Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								8 Months Ended	12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Operating revenues	$ 508.9	$ 723.2	$ 734.2	$ 901.4	$ 862.1	$ 1,113.4	$ 855.0	$ 840.7		$ 2,867.7	$ 3,671.2	$ 3,915.9
Cost of sales										1,428.9	1,918.7	2,277.9
Other operation and maintenance										489.2	601.5	581.2
Depreciation and amortization										297.3	371.0	307.1
Impairment of assets										0	0.4	6.3
Gain on insurance proceeds										0	(7.5)	(3.0)
Taxes other than income										98.8	110.2	99.7
Operating income (loss)	73.3	260.9	143.9	75.4	97.3	304.0	177.3	98.3		553.5	676.9	646.7
Equity in earnings of unconsolidated affiliates (Note 1)									101.9	101.9	0	0
Investment in unconsolidated affiliates (at historical cost)	1,298.8				0				1,298.8	1,298.8	0
Total assets	9,134.7				9,922.2				9,134.7	9,134.7	9,922.2
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues										2,262.2	2,141.2	2,211.5
Cost of sales										965.9	879.1	1,013.5
Other operation and maintenance										438.8	446.3	436.0
Depreciation and amortization										248.4	248.7	216.1
Impairment of assets											0	0
Gain on insurance proceeds											0	0
Taxes other than income										83.8	77.7	73.6
Operating income (loss)										525.3	489.4	472.3
Equity in earnings of unconsolidated affiliates (Note 1)										0
Investment in unconsolidated affiliates (at historical cost)	0								0	0
Total assets	7,694.9				7,222.4				7,694.9	7,694.9	7,222.4	6,620.9
Capital expenditures										797.6	704.4	844.5
Natural Gas Midstream Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues										630.4	1,608.6	1,787.1
Cost of sales										489.0	1,120.1	1,346.6
Other operation and maintenance										60.9	172.9	162.5
Depreciation and amortization										36.8	108.8	77.6
Impairment of assets											0.4	6.3
Gain on insurance proceeds											(7.5)	(3.0)
Taxes other than income										10.5	28.3	22.0
Operating income (loss)										33.2	185.6	175.1
Equity in earnings of unconsolidated affiliates (Note 1)										101.9
Investment in unconsolidated affiliates (at historical cost)	1,298.8								1,298.8	1,298.8
Total assets	1,348.6				2,681.3				1,348.6	1,348.6	2,681.3	2,289.0
Capital expenditures										181.5	506.5	612.5
Other Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues										0	0	0
Cost of sales										0	0	0
Other operation and maintenance										(10.5)	(17.7)	(17.3)
Depreciation and amortization										12.1	13.5	13.4
Impairment of assets											0	0
Gain on insurance proceeds											0	0
Taxes other than income										4.5	4.2	4.1
Operating income (loss)										(6.1)	0	(0.2)
Equity in earnings of unconsolidated affiliates (Note 1)										0
Investment in unconsolidated affiliates (at historical cost)	0								0	0
Total assets	216.2				242.6				216.2	216.2	242.6	155.0
Capital expenditures										11.5	18.3	13.8
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues										(24.9)	(78.6)	(82.7)
Cost of sales										(26.0)	(80.5)	(82.2)
Other operation and maintenance										0	0	0
Depreciation and amortization										0	0	0
Impairment of assets											0	0
Gain on insurance proceeds											0	0
Taxes other than income										0	0	0
Operating income (loss)										1.1	1.9	(0.5)
Equity in earnings of unconsolidated affiliates (Note 1)										0
Investment in unconsolidated affiliates (at historical cost)	0								0	0
Total assets	(125.0)				(224.1)				(125.0)	(125.0)	(224.1)	(158.9)
Capital expenditures										0	0	0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues										2,867.7	3,671.2	3,915.9
Cost of sales										1,428.9	1,918.7	2,277.9
Other operation and maintenance										489.2	601.5	581.2
Depreciation and amortization										297.3	371.0	307.1
Impairment of assets											0.4	6.3
Gain on insurance proceeds											(7.5)	(3.0)
Taxes other than income										98.8	110.2	99.7
Operating income (loss)										553.5	676.9	646.7
Investment in unconsolidated affiliates (at historical cost)	1,298.8								1,298.8	1,298.8
Total assets	9,134.7				9,922.2				9,134.7	9,134.7	9,922.2	8,906.0
Capital expenditures										$ 990.6	$ 1,229.2	$ 1,470.8