Report of Business Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Operating revenues	$ 549.9	$ 611.8	$ 1,030.0	$ 1,172.2
Cost of sales	210.9	270.9	422.5	564.3
Other operation and maintenance	113.2	111.4	224.9	223.8
Depreciation and amortization	76.2	68.3	152.1	135.5
Taxes other than income	22.4	19.4	46.9	45.0
OPERATING INCOME	127.2	141.8	183.6	203.6
Equity in earnings of unconsolidated affiliates	28.2	39.3	59.9	87.2
Other income (expense)	5.1	1.8	10.5	1.0
Interest expense	38.0	39.4	75.4	75.3
Income tax expense	35.0	42.7	47.9	66.4
Net income (loss)	87.5	100.8	130.7	150.1
Investment in unconsolidated affiliates	1,309.2	1,309.6	1,309.2	1,309.6	$ 1,318.2
Total assets	9,593.6	9,324.3	9,593.6	9,324.3	$ 9,527.8
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	549.9	611.8	1,030.0	1,172.2
Cost of sales	210.9	270.9	422.5	564.3
Other operation and maintenance	115.6	115.0	229.9	232.1
Depreciation and amortization	74.3	65.0	148.1	129.3
Taxes other than income	21.6	18.7	44.7	42.5
OPERATING INCOME	127.5	142.2	184.8	204.0
Equity in earnings of unconsolidated affiliates	0.0	0.0	0.0	0.0
Other income (expense)	4.4	1.2	7.3	1.6
Interest expense	37.3	37.5	74.1	71.4
Income tax expense	25.6	29.0	31.9	36.6
Net income (loss)	69.0	76.9	86.1	97.6
Investment in unconsolidated affiliates	0.0	0.0	0.0	0.0
Total assets	8,332.6	7,908.0	8,332.6	7,908.0
Natural Gas Midstream Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0	0.0	0.0
Cost of sales	0.0	0.0	0.0	0.0
Other operation and maintenance	0.2	0.4	1.0	0.4
Depreciation and amortization	0.0	0.0	0.0	0.0
Taxes other than income	0.0	0.0	0.0	0.0
OPERATING INCOME	(0.2)	(0.4)	(1.0)	(0.4)
Equity in earnings of unconsolidated affiliates	28.2	39.3	59.9	87.2
Other income (expense)	0.0	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	0.0
Income tax expense	10.0	14.9	18.1	33.4
Net income (loss)	18.0	24.0	40.8	53.4
Investment in unconsolidated affiliates	1,309.2	1,309.6	1,309.2	1,309.6
Total assets	1,486.6	1,371.8	1,486.6	1,371.8
Other Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0	0.0	0.0
Cost of sales	0.0	0.0	0.0	0.0
Other operation and maintenance	(2.6)	(4.0)	(6.0)	(8.7)
Depreciation and amortization	1.9	3.3	4.0	6.2
Taxes other than income	0.8	0.7	2.2	2.5
OPERATING INCOME	(0.1)	0.0	(0.2)	0.0
Equity in earnings of unconsolidated affiliates	0.0	0.0	0.0	0.0
Other income (expense)	0.8	0.6	3.3	(0.6)
Interest expense	0.8	1.9	1.4	3.9
Income tax expense	(0.6)	(1.2)	(2.1)	(3.6)
Net income (loss)	0.5	(0.1)	3.8	(0.9)
Investment in unconsolidated affiliates	0.0	0.0	0.0	0.0
Total assets	122.4	141.0	122.4	141.0
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0	0.0	0.0
Cost of sales	0.0	0.0	0.0	0.0
Other operation and maintenance	0.0	0.0	0.0	0.0
Depreciation and amortization	0.0	0.0	0.0	0.0
Taxes other than income	0.0	0.0	0.0	0.0
OPERATING INCOME	0.0	0.0	0.0	0.0
Equity in earnings of unconsolidated affiliates	0.0	0.0	0.0	0.0
Other income (expense)	(0.1)	0.0	(0.1)	0.0
Interest expense	(0.1)	0.0	(0.1)	0.0
Income tax expense	0.0	0.0	0.0	0.0
Net income (loss)	0.0	0.0	0.0	0.0
Investment in unconsolidated affiliates	0.0	0.0	0.0	0.0
Total assets	$ (348.0)	$ (96.5)	$ (348.0)	$ (96.5)