Consolidated Statements of Comprehensive Income/(Loss) (Parenthetical) - THE DAYTON POWER AND LIGHT COMPANY [Member] - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income tax (expense)/benefit on unrealized gains (losses) related to available-for-sale securities	$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ (0.2)	$ (0.1)
Other Comprehensive Income (Loss), Reclassification Adjustment for Sale of Securities Included in Net Income, Tax	0.0	0.0	0.0	0.0	0.0	0.0	0.0
AOCI reclassed to Retained Earnings, income tax	0.0	0.0	0.0	0.0
Income tax (expense)/benefit on unrealized gains (losses) related to derivative activity	0.0	0.1	(0.2)	0.1	0.1	(7.2)	(8.7)
Income tax (expense)/benefit on reclassification of earnings related to derivative activity	0.0	0.1	0.0	0.6	0.4	0.2	0.2
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives Related to Discontinued Operations, Tax	0.0	0.0	0.0	0.0	0.0	3.0	16.2
Income tax (expense)/benefit on prior service cost related to pension and postretirement activity	0.0	0.0	0.0	0.0	0.6	1.0	0.0
Income tax (expense)/benefit on net loss related to pension and postretirement activity	0.0	0.0	0.0	0.0	(0.4)	0.4	1.1
Income tax (expense)/benefit on reclassification of earnings related to pension and postretirement activity					$ (1.0)	$ (2.3)	$ (1.8)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, Tax	$ 0.9	$ (0.2)	$ (0.5)	$ (0.7)

AOCI reclassed to Retained Earnings, income tax

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Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives Related to Discontinued Operations, Tax

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No definition available.

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- Definition

Amount of tax expense (benefit) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

+ References

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- Definition

Amount of tax (expense) benefit of reclassification adjustment from accumulated other comprehensive (income) loss for prior service cost (credit) of defined benefit plan.

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- Definition

Amount of tax expense (benefit), after reclassification adjustments, of appreciation (loss) in value of unsold available-for-sale securities. Excludes amounts related to other than temporary impairment (OTTI) loss.

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Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.

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Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income of accumulated gain (loss) realized from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's deferred hedging gain (loss).

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Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan.

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Amount of tax expense (benefit) for reclassification adjustment from accumulated other comprehensive income (loss) for transition asset (obligation) of defined benefit plan.

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Amount of tax expense (benefit), before reclassification adjustments, related to increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

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