Reinsurance - Additional Information (Detail) - USD ($)		6 Months Ended
Mar. 01, 2018	Jun. 30, 2018
Ceded Credit Risk [Line Items]
Loss reinsurance, set retention amount		$ 1,000,000
Percentage of accumulation of losses		100.00%
Catastrophe reinsurance, set retention amount		$ 5,000,000
Catastrophe reinsurance, deductible amount		$ 5,000,000
Increase loss retention of reinsurance coverage	$ 250,000
Maximum [Member]
Ceded Credit Risk [Line Items]
Increase loss retention of reinsurance coverage	$ 1,000,000
Atlantic States [Member]
Ceded Credit Risk [Line Items]
Percentage of share in results of pooled business subsidiary		80.00%
Donegal Mutual [Member]
Ceded Credit Risk [Line Items]
Percentage of share in results of pooled business owned by third party		20.00%
Maximum amount of loss coverage under reinsurance agreement of property catastrophe		$ 170,000,000

Catastrophe reinsurance agreement contracts deductible amount.

No definition available.

Name:	dgica_CatastropheReinsuranceAgreementContractsDeductibleAmount
Namespace Prefix:	dgica_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Catastrophe reinsurance agreement contracts retention amount.

+ References

No definition available.

+ Details

Name:	dgica_CatastropheReinsuranceAgreementContractsRetentionAmount
Namespace Prefix:	dgica_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Loss reinsurance agreement contracts retention amount.

+ References

No definition available.

+ Details

Name:	dgica_LossReinsuranceAgreementContractsRetentionAmount
Namespace Prefix:	dgica_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Maximum amount of loss coverage under reinsurance agreement of property catastrophe.

+ References

No definition available.

+ Details

Name:	dgica_MaximumAmountOfLossCoverageUnderReinsuranceAgreementOfPropertyCatastrophe
Namespace Prefix:	dgica_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Percentage of share in results of pooled business owned by third party.

+ References

No definition available.

+ Details

Name:	dgica_PercentageOfShareInResultsOfPooledBusinessOwnedByThirdParty
Namespace Prefix:	dgica_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of share in results of pooled business subsidiary.

+ References

No definition available.

+ Details

Name:	dgica_PercentageOfShareInResultsOfPooledBusinessSubsidiary
Namespace Prefix:	dgica_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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

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

- Definition

Ratio of insurance contracts that are subject to reinsurance participation to all contracts in force.

+ References

No definition available.

+ Details

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

- Definition

Amount of risk reinsured in excess of stated retention per company policy.

+ References

No definition available.

+ Details

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

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=dgica_AtlanticStatesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=dgica_DonegalMutualMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: