Fair Value (Nonrecurring Unobservable Inputs) (Details) - USD ($)		12 Months Ended
		Dec. 31, 2017	Feb. 28, 2017	Dec. 31, 2016	Aug. 31, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Fair Value | Level 3
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets, Fair Value Disclosure, Nonrecurring		$ 64,000,000
Long Lived Assets Held And Used [Member] | Laurel Mountain [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets, Fair Value Disclosure, Nonrecurring		33,000,000
Long Lived Assets Held And Used [Member] | Buffalo Gap [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets, Fair Value Disclosure, Nonrecurring				$ 36,000,000
Long Lived Assets Held And Used [Member] | buffalo gap II [Member] [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets, Fair Value Disclosure, Nonrecurring				92,000,000
Long Lived Assets Held And Used [Member] | Kilroot [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets, Fair Value Disclosure, Nonrecurring		20,000,000						$ 70,000,000
Long Lived Assets Held And Used [Member] | Fair Value | Level 3 | Laurel Mountain [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets, Fair Value Disclosure, Nonrecurring	[1]	33,000,000
Long Lived Assets Held And Used [Member] | Fair Value | Level 3 | Buffalo Gap [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets, Fair Value Disclosure, Nonrecurring	[1]				$ 36,000,000
Long Lived Assets Held And Used [Member] | Fair Value | Level 3 | DPL Subsidiary
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets, Fair Value Disclosure, Nonrecurring	[1]		$ 11,000,000	$ 103,000,000		$ 89,000,000
Long Lived Assets Held And Used [Member] | Fair Value | Level 3 | buffalo gap II [Member] [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets, Fair Value Disclosure, Nonrecurring	[1]						$ 92,000,000
Long Lived Assets Held And Used [Member] | Fair Value | Level 3 | Kilroot [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets, Fair Value Disclosure, Nonrecurring	[1]	$ 20,000,000
Income Approach Valuation Technique [Member] | Long Lived Assets Held And Used [Member] | Minimum | Laurel Mountain [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Fair Value Inputs, Long-term Revenue Growth Rate		(30.00%)
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent		(61.00%)
Fair Value Inputs, Discount Rate		0.00%
Income Approach Valuation Technique [Member] | Long Lived Assets Held And Used [Member] | Minimum | buffalo gap II [Member] [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Fair Value Inputs, Long-term Revenue Growth Rate		(0.00%)
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent		(0.00%)
Income Approach Valuation Technique [Member] | Long Lived Assets Held And Used [Member] | Minimum | Kilroot [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Fair Value Inputs, Long-term Revenue Growth Rate		(85.00%)
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent		(31.80%)
Income Approach Valuation Technique [Member] | Long Lived Assets Held And Used [Member] | Maximum | Laurel Mountain [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Fair Value Inputs, Long-term Revenue Growth Rate		(2.00%)
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent		(73.00%)
Fair Value Inputs, Discount Rate		0.00%
Income Approach Valuation Technique [Member] | Long Lived Assets Held And Used [Member] | Maximum | buffalo gap II [Member] [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Fair Value Inputs, Long-term Revenue Growth Rate		(0.00%)
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent		(0.00%)
Income Approach Valuation Technique [Member] | Long Lived Assets Held And Used [Member] | Maximum | Kilroot [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Fair Value Inputs, Long-term Revenue Growth Rate		(17.00%)
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent		(28.00%)
Income Approach Valuation Technique [Member] | Long Lived Assets Held And Used [Member] | Weighted Average | Laurel Mountain [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Fair Value Inputs, Long-term Revenue Growth Rate		(0.00%)
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent		(64.00%)
Income Approach Valuation Technique [Member] | Long Lived Assets Held And Used [Member] | Weighted Average | DPL Subsidiary
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Fair Value Inputs, Discount Rate		7.00%
Income Approach Valuation Technique [Member] | Long Lived Assets Held And Used [Member] | Weighted Average | buffalo gap II [Member] [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Fair Value Inputs, Long-term Revenue Growth Rate		(0.00%)
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent		(0.00%)
Fair Value Inputs, Discount Rate		0.00%
Income Approach Valuation Technique [Member] | Long Lived Assets Held And Used [Member] | Weighted Average | Kilroot [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Fair Value Inputs, Long-term Revenue Growth Rate		(16.00%)
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent		(6.00%)
Fair Value Inputs, Discount Rate		8.00%

[1] See Note 19—Asset Impairment Expense for further information.