Employee benefit plans: (Details) - USD ($)	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Employee benefit plans:
Maximum percentage of eligible annual compensation that the employee can contribute subject to IRS limitations	60.00%
Percentage of employee's contribution percent matched	75.00%
Employer matching contribution, as a percent of employee's eligible compensation	6.00%
Amount of contributions to the matching feature of the 401(k) plan	$ 1,371,000	$ 1,310,000	$ 1,205,000
Contribution to employer retirement contribution feature (as a percent)	11.00%	8.00%
Amount of contributions to the employer retirement contribution feature of the 401(k) plan	$ 3,678,000	$ 2,611,000	$ 2,441,000

Discretionary contribution percentage made by an employer to a defined contribution plan.

No definition available.

Name:	opc_DefinedContributionPlanEmployerDiscretionaryContributionPercent
Namespace Prefix:	opc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Maximum percentage employer matches of the employee's percentage contribution matched.

No definition available.

Name:	opc_DefinedContributionPlanEmployerMatchingContributionPercentOfMatchMaximum
Namespace Prefix:	opc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Represents the amount of contribution made by an employer to the matching feature of a defined contribution plan.

No definition available.

Name:	opc_DefinedContributionPlanEmployersMatchingContributionAmount
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

No definition available.

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

Amount of discretionary contributions made by an employer to a defined contribution plan.

No definition available.

Name:	us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

No definition available.

Name:	us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

No definition available.

Name:	us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration