Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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- Definition

Represents the cash settlement value if fixed rate is at or above the swap rate upon expiration.

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

Represents the aggregate notional amount expired, specified by the derivative(s), expressed as an absolute value.

+ References

No definition available.

+ Details

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- Definition

The number of derivative instruments that have expired.

+ References

No definition available.

+ Details

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- Definition

The number of derivative instruments originally purchased.

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
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- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Represents the number of nuclear units expected to be financed.

+ References

No definition available.

+ Details

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Balance Type:	na
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- 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

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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

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Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: 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 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=49121117&loc=d3e13433-108611

Reference 4: 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 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13495-108611

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226008-175313

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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

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

- Definition

The percentage points added to the reference rate to compute the variable rate on the interest rate derivative.

+ References

No definition available.

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

The number of derivative instruments of a particular group held by the entity.

+ References

Reference 1: 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=56946850&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5579245-113959

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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

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

- Details

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- Details

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- Details

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Derivative Instruments - Interest rate options (Details)	3 Months Ended	9 Months Ended
	Dec. 31, 2011 USD ($) item	Sep. 30, 2016 USD ($) item	Dec. 31, 2015 USD ($)
LIBOR
Derivative instruments and hedging activities
Variable rate basis		LIBOR
Interest rate options
Derivative instruments and hedging activities
Number of derivative instruments originally purchased \| item	17
Purchased amount of derivative instrument	$ 100,000,000
Notional Dollar Amount	$ 2,200,000,000	$ 159,835,000
Number of nuclear units expected to be financed \| item	2
Number of derivative instruments that have expired \| item		15
Derivative notional amount expired		$ 2,019,368,000
Number of derivative instruments \| item		2
Cash settlement value if swaption is out of money		$ 0
Notional amount of derivatives, expired without value		230,867,000
Fair value of assets		500	$ 1,010,000
Funds posted as collateral of the counterparties		0	$ 0
Interest rate options \| 2016
Derivative instruments and hedging activities
Notional Dollar Amount		79,666,000
Interest rate options \| 2017
Derivative instruments and hedging activities
Notional Dollar Amount		80,169,000
Interest rate options \| Minimum
Derivative instruments and hedging activities
Collateral thresholds range		0
Interest rate options \| Maximum
Derivative instruments and hedging activities
Collateral thresholds range		10,000,000
Interest rate options \| Department of Energy-guaranteed loan
Derivative instruments and hedging activities
Hedged amount of expected debt		$ 2,200,000,000
Interest Rate Swaption Expiring December 2016
Derivative instruments and hedging activities
Number of derivative instruments \| item		1
Weighted average fixed rate (as a percent)		3.845%
Interest Rate Swaption Expiring December 2016 \| LIBOR
Derivative instruments and hedging activities
Basis spread (as a percent)		2.19%
Interest Rate Swaption Expiring March 2017
Derivative instruments and hedging activities
Weighted average fixed rate (as a percent)		3.8775%
Interest Rate Swaption Expiring March 2017 \| LIBOR
Derivative instruments and hedging activities
Basis spread (as a percent)		2.23%