Business Segments (Narrative) (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 mi² customer segment	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Service area, square miles	6,000
THE DAYTON POWER AND LIGHT COMPANY [Member]
Segment Reporting Information [Line Items]
Number of Operating Segments \| segment	2
Approximate number of retail customers \| customer	521,000
Service area, square miles	6,000
Transmission and Distribution [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
OVEC Revenue \| $		$ 19.7

Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration