Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income (loss)	$ (94.6)	$ (485.2)	$ (239.0)
Adjustments to reconcile Net loss to Net cash from operating activities
Depreciation and amortization	106.9	132.3	138.8
Amortization of debt market value adjustments	0.1	0.1	(1.1)
Amortization of deferred financing costs	3.6	5.6	5.9
Unrealized loss (gain) on derivatives	(1.7)	(4.3)	5.8
Deferred income taxes	(22.2)	(306.2)	(17.1)
Charge for early redemption of debt	3.3	3.1	2.1
Goodwill impairment	0.0	0.0	317.0
Fixed-asset impairment (Note 15)	175.8	859.0	0.0
Loss / (Gain) on asset disposal	2.2	(49.2)	0.4
Changes in certain assets and liabilities:
Accounts receivable	16.8	24.2	43.4
Inventories	7.7	32.0	(9.0)
Prepaid taxes	0.0	0.2	(1.3)
Taxes applicable to subsequent years	2.3	0.2	(3.4)
Deferred regulatory costs, net	(23.7)	4.1	21.8
Accounts payable	(35.0)	16.5	(5.1)
Accrued taxes payable	(3.7)	45.1	43.8
Accrued interest payable	(1.3)	(3.7)	(5.7)
Pension, retiree and other benefits	4.5	8.6	(0.7)
Unamortized investment tax credit	(0.3)	(0.4)	(0.5)
Insurance and claims costs	(2.4)	(0.5)	(0.5)
Other	(6.6)	(14.4)	12.9
Net cash provided by operating activities	131.7	267.1	308.5
Cash flows from investing activities:
Capital expenditures	(121.5)	(148.5)	(137.2)
Proceeds from sale of business	70.1	75.5	1.3
Insurance proceeds	12.3	6.3	0.0
Purchase of renewable energy credits	(0.6)	(0.4)	(0.8)
Decrease / (increase) in restricted cash	27.1	(11.8)	(0.4)
Other investing activities, net	0.4	1.1	0.4
Net cash used in investing activities	(12.2)	(77.8)	(136.7)
Cash flows from financing activities:
Deferred financing costs	0.0	(8.6)	(6.9)
Preferred Stock, Redemption Amount	0.0	(23.5)	0.0
Retirement of debt	(159.5)	(577.8)	(474.5)
Premium paid for early redemption of debt	(0.1)	0.0	0.0
Issuance of long-term debt	0.0	442.8	325.0
Borrowings from revolving credit facilities	102.5	15.0	80.0
Repayment of borrowings from revolving credit facilities	(92.5)	(15.0)	(80.0)
Net cash from financing activities	(149.6)	(167.1)	(156.4)
Cash and cash equivalents:
Net increase / (decrease) in cash	(30.1)	22.2	15.4
Balance at beginning of period	54.6	32.4	17.0
Cash and cash equivalents at end of period	24.5	54.6	32.4
Supplemental cash flow information:
Interest paid, net of amounts capitalized	105.2	103.8	111.6
Income taxes paid / (refunded), net	0.0	0.3	0.8
Non-cash financing and investing activities:
Accruals for capital expenditures	12.9	16.2	18.6
Non-cash capital contribution	97.1	0.0	0.0
THE DAYTON POWER AND LIGHT COMPANY [Member]
Cash flows from operating activities:
Net income (loss)	17.0	(772.7)	106.4
Adjustments to reconcile Net loss to Net cash from operating activities
Depreciation and amortization	87.2	120.3	138.2
Amortization of deferred financing costs	1.1	2.9	2.9
Unrealized loss (gain) on derivatives	(1.0)	(4.2)	5.7
Deferred income taxes	8.1	(477.5)	(19.2)
Charge for early redemption of debt	1.1	0.5	4.8
Fixed-asset impairment (Note 15)	66.3	1,353.5	0.0
Loss / (Gain) on asset disposal	15.7	0.0	0.4
Changes in certain assets and liabilities:
Accounts receivable	13.3	(9.7)	28.7
Inventories	10.3	32.2	(9.1)
Prepaid taxes	0.0	2.7	(1.3)
Taxes applicable to subsequent years	6.4	0.0	(3.7)
Deferred regulatory costs, net	(23.7)	4.1	21.8
Accounts payable	(48.0)	16.0	(5.8)
Accrued taxes payable	(17.5)	(10.5)	7.3
Accrued interest payable	(1.3)	(2.0)	(5.7)
Pension, retiree and other benefits	4.3	8.6	(0.7)
Unamortized investment tax credit	(1.7)	(2.3)	(2.4)
Other	(2.2)	2.9	(11.6)
Net cash provided by operating activities	135.4	264.8	256.7
Cash flows from investing activities:
Capital expenditures	(101.7)	(128.3)	(127.0)
Insurance proceeds	12.5	6.1	5.2
Purchase of renewable energy credits	(0.6)	(0.4)	(0.8)
Decrease / (increase) in restricted cash	26.6	(11.9)	(0.3)
Other investing activities, net	0.3	1.1	0.4
Net cash used in investing activities	(62.9)	(133.4)	(122.5)
Cash flows from financing activities:
Dividends paid on preferred stock	0.0	(0.7)	(0.9)
Deferred financing costs	0.0	(8.5)	(3.9)
Preferred Stock, Redemption Amount	0.0	(23.5)	0.0
Retirement of debt	(104.5)	(445.3)	(314.4)
Proceeds from Contributions from Parent	70.0	0.0	0.0
Premium paid for early redemption of debt	(0.4)	0.0	0.0
Issuance of long-term debt	0.0	442.8	200.0
Borrowings from revolving credit facilities	40.0	0.0	50.0
Repayment of borrowings from revolving credit facilities	(30.0)	0.0	(50.0)
Dividends paid on common stock to parent	(39.0)	(70.0)	(50.0)
Borrowings from related party	30.0	10.0	35.0
Repayment of borrowings from related party	(35.0)	(40.0)	0.0
Net cash from financing activities	(68.9)	(135.2)	(134.2)
Cash and cash equivalents:
Net increase / (decrease) in cash	3.6	(3.8)	0.0
Balance at beginning of period	1.6	5.4	5.4
Cash and cash equivalents at end of period	5.2	1.6	5.4
Supplemental cash flow information:
Interest paid, net of amounts capitalized	28.4	21.4	27.5
Income taxes paid / (refunded), net	28.1	0.3	0.8
Non-cash financing and investing activities:
Accruals for capital expenditures	19.7	14.8	16.9
Subsidiary of Common Parent [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Non-cash financing and investing activities:
Equity Settlement of Related Party Payable	$ 0.0	$ 7.5	$ 0.0