Represents the carrying amount of derivative assets as of the balance sheet date.

No definition available.

Name:	opc_DerivativeAssetsCarryingAmount
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the cash settlement value if swap rates are at or below the fixed rate upon expiration.

+ References

No definition available.

+ Details

Name:	opc_DerivativeCashSettlementValueIfSwapRatesAreAtOrBelowFixedRateUponExpiration
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the amount of funds posted as collateral by the counterparties.

+ References

No definition available.

+ Details

Name:	opc_DerivativeCollateralFromCounterparties
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the threshold of collateral for derivative instruments to be received from counterparties.

+ References

No definition available.

+ Details

Name:	opc_DerivativeCollateralThreshold
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the fixed interest rate on the derivative instrument above the current swap rate.

+ References

No definition available.

+ Details

Name:	opc_DerivativeFixedInterestRateAboveCurrentSwapRate
Namespace Prefix:	opc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the additional amount of premium payment obligations on interest rate option transactions.

+ References

No definition available.

+ Details

Name:	opc_DerivativePremiumAdditionalPaymentObligations
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the notional amount of interest rate derivatives that expired without value.

+ References

No definition available.

+ Details

Name:	opc_NotionalAmountOfInterestRateDerivativesThatExpiredWithoutValue
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the number of derivative instruments expired without value, during the period.

+ References

No definition available.

+ Details

Name:	opc_NumberOfDerivativeInstrumentsExpiredWithoutValue
Namespace Prefix:	opc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of nuclear units expected to be financed.

+ References

No definition available.

+ Details

Name:	opc_NumberOfNuclearUnitsExpectedToBeFinanced
Namespace Prefix:	opc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentDescriptionOfVariableRateBasis
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of the hedged item as of the balance sheet date related to the derivative. For example, the hedged balance on a debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeAmountOfHedgedItem
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FIN39-1

-Paragraph 10A, 10B

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13433-108611

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 4, 17

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13495-108611

+ Details

Name:	us-gaap_DerivativeAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Average fixed interest rate that will be received or paid upon exercise of a group of interest rate swaption contracts.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeAverageSwaptionInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_DerivativeLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate notional amount of interest rate derivatives, which relates to the currency amount specified in the interest rate derivative instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1A

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5579245-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Notional Amount

-URI http://asc.fasb.org/extlink&oid=6519104

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1B

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5580258-113959

+ Details

Name:	us-gaap_NotionalAmountOfInterestRateDerivatives
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The cash outflow for derivative instruments during the period, which are classified as investing activities, excluding those designated as hedging instruments.

+ References

No definition available.

+ Details

Name:	us-gaap_PaymentsForDerivativeInstrumentInvestingActivities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Derivative Instruments (Details 2) (USD $)	1 Months Ended	3 Months Ended
Derivative Instruments (Details 2) (USD $)	Mar. 31, 2013 item	Mar. 31, 2013	Dec. 31, 2011	Dec. 31, 2012
Derivative instruments and hedging activities
Variable rate basis		LIBOR
Cash settlement value if swap rates are at or below the fixed rate upon expiration	$ 0	$ 0
Additional premium payment obligations on interest rate option transactions	0	0
Department of Energy-guaranteed loan
Derivative instruments and hedging activities
Number of nuclear units expected to be financed			2
LIBOR swaptions
Derivative instruments and hedging activities
Purchased amount of derivative instrument			100,000,000
Variable rate basis		LIBOR
Weighted average fixed rate (as a percent)	4.17%	4.17%
Number of derivative instruments expired without value	1
Notional amount of derivatives,expired without value	185,832,000	185,832,000
Fair value of assets	26,539,000	26,539,000		25,783,000
Funds posted as collateral by the counterparties	13,360,000	13,360,000		8,950,000
Carrying amount of derivative assets	13,179,000	13,179,000		16,833,000
Notional Dollar Amount	1,993,373,000	1,993,373,000
LIBOR swaptions \| 2013
Derivative instruments and hedging activities
Notional Dollar Amount	568,621,000	568,621,000
LIBOR swaptions \| 2014
Derivative instruments and hedging activities
Notional Dollar Amount	563,425,000	563,425,000
LIBOR swaptions \| 2015
Derivative instruments and hedging activities
Notional Dollar Amount	470,625,000	470,625,000
LIBOR swaptions \| 2016
Derivative instruments and hedging activities
Notional Dollar Amount	310,533,000	310,533,000
LIBOR swaptions \| 2017
Derivative instruments and hedging activities
Notional Dollar Amount	80,169,000	80,169,000
LIBOR swaptions \| Minimum
Derivative instruments and hedging activities
Fixed interest rates above current LIBOR swap rates (as a percent)	1.00%	1.00%
Collateral thresholds range	0	0
LIBOR swaptions \| Maximum
Derivative instruments and hedging activities
Fixed interest rates above current LIBOR swap rates (as a percent)	2.00%	2.00%
Collateral thresholds range	10,000,000	10,000,000
LIBOR swaptions \| Department of Energy-guaranteed loan
Derivative instruments and hedging activities
Hedged amount of expected debt	$ 2,000,000,000	$ 2,000,000,000	$ 2,200,000,000