Report of Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Segment Reporting Information [Line Items]
Operating revenues	$ 560.4	$ 901.4
Cost of sales	293.4	553.0
Other operation and maintenance	112.4	148.0
Depreciation and amortization	67.2	91.9
Taxes other than income	25.6	33.1
Operating income (loss)	61.8	75.4
Equity in earnings of unconsolidated affiliates	47.9	0
Investment in unconsolidated affiliates	1,314.2		1,298.8
Total assets	9,250.7	9,981.8	9,134.7
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	560.4	455.5
Cost of sales	293.4	213.0
Other operation and maintenance	117.1	105.1
Depreciation and amortization	64.3	61.3
Taxes other than income	23.8	23.2
Operating income (loss)	61.8	52.9
Equity in earnings of unconsolidated affiliates	0
Investment in unconsolidated affiliates	0
Total assets	7,826.8	7,138.4
Natural Gas Midstream Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	464.3
Cost of sales	0	359.2
Other operation and maintenance	0	45.2
Depreciation and amortization	0	27.6
Taxes other than income	0	8.0
Operating income (loss)	0	24.3
Equity in earnings of unconsolidated affiliates	47.9
Investment in unconsolidated affiliates	1,314.2
Total assets	1,367.4	2,729.9
Other Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0
Cost of sales	0	0
Other operation and maintenance	(4.7)	(2.3)
Depreciation and amortization	2.9	3.0
Taxes other than income	1.8	1.9
Operating income (loss)	0	(2.6)
Equity in earnings of unconsolidated affiliates	0
Investment in unconsolidated affiliates	0
Total assets	127.9	381.6
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	(18.4)
Cost of sales	0	(19.2)
Other operation and maintenance	0	0
Depreciation and amortization	0	0
Taxes other than income	0	0
Operating income (loss)	0	0.8
Equity in earnings of unconsolidated affiliates	0
Investment in unconsolidated affiliates	0
Total assets	$ (71.4)	$ (268.1)