Contingencies and Regulatory Matters (Details) (Plant Vogtle)	6 Months Ended
Jun. 30, 2012 MW item
Plant Vogtle
Nuclear Construction
Number of Westinghouse AP1000 nuclear generating units	2
Nominally rated generating capacity for each unit	1,100
Ownership interest (as a percent)	30.00%
Total generating capacity owned	660

Represents the generation capacity of the nuclear plant owned by the entity.

No definition available.

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

- Details

Name:	opc_JointlyOwnedUtilityPlantNameLineItems
Namespace Prefix:	opc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the nominally rated generating capacity of the nuclear units.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of units constructed in connection with the generation of nuclear energy.

+ References

No definition available.

+ Details

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

- Definition

The proportionate share of interest (percent) in a jointly owned utility plant.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 360

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 10.C)

-URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033

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

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 10

-Section C

+ Details

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