Consolidated Statements of Revenues and Expenses (Unaudited) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Operating revenues:
Total operating revenues	$ 589,470	$ 633,748	$ 1,326,526	$ 1,311,324
Operating expenses:
Fuel	173,274	186,838	458,665	429,485
Production	132,069	157,956	271,856	288,284
Depreciation and amortization	111,444	107,341	222,052	213,598
Purchased power	23,305	22,109	46,000	43,840
Accretion	14,794	14,607	29,506	29,139
Total operating expenses	454,886	488,851	1,028,079	1,004,346
Operating margin	134,584	144,897	298,447	306,978
Other income:
Investment income	7,569	8,047	14,730	16,544
Other	3,143	492	8,634	4,051
Total other income	10,712	8,539	23,364	20,595
Interest charges:
Interest expense	136,778	135,537	271,477	267,971
Allowance for debt funds used during construction	(10,187)	(4,991)	(19,001)	(8,719)
Amortization of debt discount and expense	2,481	2,595	4,962	5,020
Net interest charges	129,072	133,141	257,438	264,272
Net margin	16,224	20,295	64,373	63,301
Sales to members
Operating revenues:
Total operating revenues	574,157	601,690	1,281,158	1,269,991
Sales to non-members
Operating revenues:
Total operating revenues	$ 15,313	$ 32,058	$ 45,368	$ 41,333

Amount of allowance for debt funds used during construction.

No definition available.

Name:	opc_AllowanceForDebtFundsUsedDuringConstruction
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This item represents investment income derived from investments in debt and equity securities consisting of interest income earned from investments in debt securities and on cash and cash equivalents, dividend income from investments in equity securities, and income or expense derived from the amortization of investment related discounts or premiums, respectively, net of related investment expenses. It also includes the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain or loss of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain or loss which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains or losses realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments of the subject investments.

+ References

No definition available.