Report of Business Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Operating revenues	$ 433.1	$ 480.1
Cost of sales	177.9	211.6
Other operation and maintenance	113.9	111.7
Depreciation and amortization	78.5	75.9
Taxes other than income	24.9	24.5
OPERATING INCOME	37.9	56.4
Equity in earnings of unconsolidated affiliates	28.3	31.7
Other income (expense)	5.5	5.4
Interest expense	36.3	37.4
Income tax expense (benefit)	10.2	12.9
NET INCOME	25.2	43.2
Investment in unconsolidated affiliates	1,187.4	1,315.7	$ 1,194.4
Total assets	9,523.3	9,484.3	$ 9,580.6
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	433.1	480.1
Cost of sales	177.9	211.6
Other operation and maintenance	116.3	114.3
Depreciation and amortization	76.7	73.8
Taxes other than income	23.6	23.1
OPERATING INCOME	38.6	57.3
Equity in earnings of unconsolidated affiliates	0.0	0.0
Other income (expense)	5.3	2.9
Interest expense	35.5	36.8
Income tax expense (benefit)	2.3	6.3
NET INCOME	6.1	17.1
Investment in unconsolidated affiliates	0.0	0.0
Total assets	8,288.5	8,217.7
Natural Gas Midstream Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0
Cost of sales	0.0	0.0
Other operation and maintenance	0.2	0.8
Depreciation and amortization	0.0	0.0
Taxes other than income	0.0	0.0
OPERATING INCOME	(0.2)	(0.8)
Equity in earnings of unconsolidated affiliates	28.3	31.7
Other income (expense)	0.0	0.0
Interest expense	0.0	0.0
Income tax expense (benefit)	10.1	8.1
NET INCOME	18.0	22.8
Investment in unconsolidated affiliates	1,187.4	1,315.7
Total assets	1,467.9	1,476.7
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	(2.6)	(3.4)
Depreciation and amortization	1.8	2.1
Taxes other than income	1.3	1.4
OPERATING INCOME	(0.5)	(0.1)
Equity in earnings of unconsolidated affiliates	0.0	0.0
Other income (expense)	0.3	2.5
Interest expense	0.9	0.6
Income tax expense (benefit)	(2.2)	(1.5)
NET INCOME	1.1	3.3
Investment in unconsolidated affiliates	0.0	0.0
Total assets	98.7	122.3
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0
Cost of sales	0.0	0.0
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	0.0	0.0
Equity in earnings of unconsolidated affiliates	0.0	0.0
Other income (expense)	(0.1)	0.0
Interest expense	(0.1)	0.0
Income tax expense (benefit)	0.0	0.0
NET INCOME	0.0	0.0
Investment in unconsolidated affiliates	0.0	0.0
Total assets	$ (331.8)	$ (332.4)