Report of Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Operating revenues	$ 480.1us-gaap_Revenues	$ 560.4us-gaap_Revenues
Cost of sales	211.6us-gaap_CostOfGoodsAndServicesSold	293.4us-gaap_CostOfGoodsAndServicesSold
Other operation and maintenance	111.7us-gaap_OtherCostAndExpenseOperating	112.4us-gaap_OtherCostAndExpenseOperating
Depreciation and amortization	75.9us-gaap_DepreciationAndAmortization	67.2us-gaap_DepreciationAndAmortization
Taxes other than income	24.5	25.6
OPERATING INCOME	56.4us-gaap_OperatingIncomeLoss	61.8us-gaap_OperatingIncomeLoss
Equity in earnings of unconsolidated affiliates	31.7	47.9
Other income (expense)	5.4us-gaap_OtherNonoperatingIncomeExpense	(0.8)us-gaap_OtherNonoperatingIncomeExpense
Interest expense	37.4us-gaap_InterestExpense	35.9us-gaap_InterestExpense
Income tax expense	12.9us-gaap_IncomeTaxExpenseBenefit	23.7us-gaap_IncomeTaxExpenseBenefit
Net income (loss)	43.2us-gaap_ProfitLoss	49.3us-gaap_ProfitLoss
Investment in unconsolidated affiliates	1,315.7us-gaap_EquityMethodInvestments	1,314.2us-gaap_EquityMethodInvestments	1,318.2us-gaap_EquityMethodInvestments
Total assets	9,502.2us-gaap_Assets	9,250.7us-gaap_Assets	9,527.8us-gaap_Assets
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	480.1	560.4
Cost of sales	211.6	293.4
Other operation and maintenance	114.3	117.1
Depreciation and amortization	73.8	64.3
Taxes other than income	23.1	23.8
OPERATING INCOME	57.3	61.8
Equity in earnings of unconsolidated affiliates	0	0
Other income (expense)	2.9	0.4
Interest expense	36.8	33.9
Income tax expense	6.3	7.6
Net income (loss)	17.1	20.7
Investment in unconsolidated affiliates	0	0
Total assets	8,234.9	7,826.8
Natural Gas Midstream Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0
Cost of sales	0	0
Other operation and maintenance	0.8	0
Depreciation and amortization	0	0
Taxes other than income	0	0
OPERATING INCOME	(0.8)	0
Equity in earnings of unconsolidated affiliates	31.7	47.9
Other income (expense)	0	0
Interest expense	0	0
Income tax expense	8.1	18.5
Net income (loss)	22.8	29.4
Investment in unconsolidated affiliates	1,315.7	1,314.2
Total assets	1,476.7	1,367.4
Other Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0
Cost of sales	0	0
Other operation and maintenance	(3.4)	(4.7)
Depreciation and amortization	2.1	2.9
Taxes other than income	1.4	1.8
OPERATING INCOME	(0.1)	0
Equity in earnings of unconsolidated affiliates	0	0
Other income (expense)	2.5	(1.2)
Interest expense	0.6	2.0
Income tax expense	(1.5)	(2.4)
Net income (loss)	3.3	(0.8)
Investment in unconsolidated affiliates	0	0
Total assets	123.0	127.9
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0
Cost of sales	0	0
Other operation and maintenance	0	0
Depreciation and amortization	0	0
Taxes other than income	0	0
OPERATING INCOME	0	0
Equity in earnings of unconsolidated affiliates	0	0
Other income (expense)	0	0
Interest expense	0	0
Income tax expense	0	0
Net income (loss)	0	0
Investment in unconsolidated affiliates	0	0
Total assets	$ (332.4)	$ (71.4)

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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All taxes not related to income of the entity or excise or sales taxes levied on the revenue of the entity that are not reported elsewhere. These taxes could include production, real estate, personal property, and pump tax.

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