Report of Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Segment Reporting Information [Line Items]
Operating revenues	$ 611.8	$ 734.2	$ 1,172.2	$ 1,635.6
Cost of sales	270.9	370.6	564.3	923.6
Other operation and maintenance	111.4	122.0	223.8	270.0
Depreciation and amortization	68.3	74.4	135.5	166.3
Taxes other than income	19.4	23.3	45.0	56.4
Operating income (loss)	141.8	143.9	203.6	219.3
Equity in earnings of unconsolidated affiliates	39.3	18.5	87.2	18.5
Investment in unconsolidated affiliates	1,309.6	1,267.2	1,309.6	1,267.2	1,298.8
Total assets	9,324.3	8,848.5	9,324.3	8,848.5	9,134.7
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	611.8	574.6	1,172.2	1,030.1
Cost of sales	270.9	247.6	564.3	460.6
Other operation and maintenance	115.0	107.0	232.1	212.1
Depreciation and amortization	65.0	62.0	129.3	123.3
Taxes other than income	18.7	19.9	42.5	43.1
Operating income (loss)	142.2	138.1	204.0	191.0
Equity in earnings of unconsolidated affiliates	0	0	0	0
Investment in unconsolidated affiliates	0	0	0	0
Total assets	7,908.0	7,453.6	7,908.0	7,453.6
Natural Gas Midstream Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	166.1	0	630.4
Cost of sales	0	129.7	0	489.0
Other operation and maintenance	0.4	15.7	0.4	60.9
Depreciation and amortization	0	9.2	0	36.8
Taxes other than income	0	2.5	0	10.5
Operating income (loss)	(0.4)	9.0	(0.4)	33.2
Equity in earnings of unconsolidated affiliates	39.3	18.5	87.2	18.5
Investment in unconsolidated affiliates	1,309.6	1,267.2	1,309.6	1,267.2
Total assets	1,371.8	1,266.8	1,371.8	1,266.8
Other Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Cost of sales	0	0	0	0
Other operation and maintenance	(4.0)	(0.7)	(8.7)	(3.0)
Depreciation and amortization	3.3	3.2	6.2	6.2
Taxes other than income	0.7	0.9	2.5	2.8
Operating income (loss)	0	(3.4)	0	(6.0)
Equity in earnings of unconsolidated affiliates	0	0	0	0
Investment in unconsolidated affiliates	0	0	0	0
Total assets	141.0	189.4	141.0	189.4
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	(6.5)	0	(24.9)
Cost of sales	0	(6.7)	0	(26.0)
Other operation and maintenance	0	0	0	0
Depreciation and amortization	0	0	0	0
Taxes other than income	0	0	0	0
Operating income (loss)	0	0.2	0	1.1
Equity in earnings of unconsolidated affiliates	0	0	0	0
Investment in unconsolidated affiliates	0	0	0	0
Total assets	$ (96.5)	$ (61.3)	$ (96.5)	$ (61.3)