Fair Value (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Long-term debt, earliest maturities	2019
Long-term debt, latest maturities	2061
Unrealized gains in AOCI, before tax	$ 1.2
Unrealized Gains in AOCI, Net of Tax	0.8
Unrealized Gains and Immaterial Unrealized Losses in AOCI, Before Tax		$ 1.0
Unrealized gains and immaterial unrealized losses in AOCI, after tax		0.6
Realized gains on investments sold to facilitate the distribution of benefits	0.7
Realized gains on investments sold to facilitate the distribution of benefits, after tax	$ 0.5
Percent of inputs to the fair value of derivative instruments from quoted market prices (percent)	94.00%
Increase (Decrease) in Asset Retirement Obligations	$ 4.4
THE DAYTON POWER AND LIGHT COMPANY [Member]
Long-term debt, earliest maturities	2020
Long-term debt, latest maturities	2061
Unrealized gains in AOCI, before tax	$ 1.3
Unrealized Gains in AOCI, Net of Tax	0.9
Unrealized Gains and Immaterial Unrealized Losses in AOCI, Before Tax		1.1
Unrealized gains and immaterial unrealized losses in AOCI, after tax		$ 0.7
Realized gains on investments sold to facilitate the distribution of benefits	0.7
Realized gains on investments sold to facilitate the distribution of benefits, after tax	$ 0.5
Percent of inputs to the fair value of derivative instruments from quoted market prices (percent)	94.00%
Increase (Decrease) in Asset Retirement Obligations	$ 4.4