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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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The period end amount owned by the reporting utility of a jointly owned utility plant in service.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 10.C)
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-Name Staff Accounting Bulletin (SAB)
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The proportionate share of interest (percent) in a jointly owned utility plant.

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Property, Plant and Equipment (Details) $ in Millions	3 Months Ended
Property, Plant and Equipment (Details) $ in Millions	Mar. 31, 2017 USD ($) generating_facility power_plant MW	Mar. 31, 2016 USD ($)	Apr. 21, 2017 USD ($)	Dec. 31, 2016 USD ($)
Property, Plant and Equipment [Line Items]
Asset Retirement Obligation	$ 135.6			$ 138.8
Asset Retirement Obligation, Revision of Estimate	(4.4)
Asset Retirement Obligation, Accretion Expense	1.1
Asset Retirement Obligation, Liabilities Settled	0.1
Gain (Loss) on Sale of Assets and Asset Impairment Charges	(19.4)	$ (0.1)
Electric Transmission [Member] \| Dpandl Investment [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	43.0
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	11.0
Total Jointly Owned Stations [Member] \| Dpandl Investment [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	11.0
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	91.0
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	$ 15.0
Total Jointly Owned Stations [Member] \| Dpandl Share [Member]
Property, Plant and Equipment [Line Items]
Production Plan Capacity \| MW	2,078
Conesville Unit Four [Member] \| Dpandl Investment [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	$ 0.0
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	0.0
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	$ 0.0
Conesville Unit Four [Member] \| Dpandl Share [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	16.50%
Killen Station [Member] \| Dpandl Investment [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	$ 1.0
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	7.0
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	$ 1.0
Killen Station [Member] \| Dpandl Share [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	67.00%
Miami Fort Unit Seven And Eight [Member] \| Dpandl Investment [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	$ 5.0
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	28.0
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	$ 1.0
Miami Fort Unit Seven And Eight [Member] \| Dpandl Share [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	36.00%
Stuart Station [Member] \| Dpandl Investment [Member]
Property, Plant and Equipment [Line Items]
Write off of net book value	$ 6.4
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	0.0
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	1.0
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	1.0
Gain (Loss) on Sale of Assets and Asset Impairment Charges	$ 3.2
Stuart Station [Member] \| Dpandl Share [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	35.00%
Zimmer Station [Member] \| Dpandl Investment [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	$ 5.0
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	12.0
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	$ 1.0
Zimmer Station [Member] \| Dpandl Share [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	28.10%
Subsidiaries [Member]
Property, Plant and Equipment [Line Items]
Number Of Coal Fired Power Plants \| power_plant	5
Asset Retirement Obligation	$ 132.3			$ 135.2
Asset Retirement Obligation, Revision of Estimate	$ (4.4)
Number Of Generating Facilities \| generating_facility	5
Asset Retirement Obligation, Accretion Expense	$ 1.4
Asset Retirement Obligation, Liabilities Settled	0.1
Gain (Loss) on Sale of Assets and Asset Impairment Charges	(19.4)	$ (0.1)
Subsidiaries [Member] \| Electric Transmission [Member] \| Dpandl Investment [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	99.0
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	67.0
Subsidiaries [Member] \| Total Jointly Owned Stations [Member] \| Dpandl Investment [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	11.0
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	147.0
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	$ 71.0
Subsidiaries [Member] \| Total Jointly Owned Stations [Member] \| Dpandl Share [Member]
Property, Plant and Equipment [Line Items]
Production Plan Capacity \| MW	2,078
Subsidiaries [Member] \| Conesville Unit Four [Member] \| Dpandl Investment [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	$ 0.0
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	0.0
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	$ 0.0
Subsidiaries [Member] \| Conesville Unit Four [Member] \| Dpandl Share [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	16.50%
Production Plan Capacity \| MW	129
Subsidiaries [Member] \| Killen Station [Member] \| Dpandl Investment [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	$ 1.0
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	7.0
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	$ 1.0
Subsidiaries [Member] \| Killen Station [Member] \| Dpandl Share [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	67.00%
Production Plan Capacity \| MW	402
Subsidiaries [Member] \| Miami Fort Unit Seven And Eight [Member] \| Dpandl Investment [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	$ 5.0
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	28.0
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	$ 1.0
Subsidiaries [Member] \| Miami Fort Unit Seven And Eight [Member] \| Dpandl Share [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	36.00%
Production Plan Capacity \| MW	368
Subsidiaries [Member] \| Stuart Station [Member] \| Dpandl Investment [Member]
Property, Plant and Equipment [Line Items]
Write off of net book value	$ 6.4
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	0.0
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	1.0
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	1.0
Gain (Loss) on Sale of Assets and Asset Impairment Charges	$ 3.2
Subsidiaries [Member] \| Stuart Station [Member] \| Dpandl Share [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	35.00%
Production Plan Capacity \| MW	808
Subsidiaries [Member] \| Zimmer Station [Member] \| Dpandl Investment [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	$ 5.0
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	12.0
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	$ 1.0
Subsidiaries [Member] \| Zimmer Station [Member] \| Dpandl Share [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	28.10%
Production Plan Capacity \| MW	371
Subsequent Event [Member] \| Miami Fort and Zimmer [Member] \| Dpandl Investment [Member]
Property, Plant and Equipment [Line Items]
Sale of undivided interest in jointly-owned production units			$ 50.0
Subsequent Event [Member] \| Subsidiaries [Member] \| Miami Fort and Zimmer [Member] \| Dpandl Investment [Member]
Property, Plant and Equipment [Line Items]
Sale of undivided interest in jointly-owned production units			$ 50.0