Subsequent Events (Details) - Performance Shares - Subsequent Event $ / shares in Units, shares in Millions	Sep. 30, 2020 USD ($) $ / shares shares
2018
Subsequent Event [Line Items]
Percentage of awards issued	100.00%
Period of performance	3 years
Percentage of growth in earnings	50.00%
Percentage of relative total stockholder return	50.00%
Award initial measurement period	2 years
Performance percentage	66.67%
Number of shares affected \| shares	0.2
Fair value of shares affected	$ 0
Fair value of shares affected (in dollars per share) \| $ / shares	$ 10.00
Percentage of target	100.00%
Remaining term	3 months
2018 \| Maximum
Subsequent Event [Line Items]
Fair value of shares affected	$ 2,400,000
2019
Subsequent Event [Line Items]
Percentage of awards issued	100.00%
Period of performance	3 years
Percentage of growth in earnings	50.00%
Percentage of relative total stockholder return	50.00%
Number of shares affected \| shares	0.3
Fair value of shares affected	$ 0
Remaining term	15 months
2019 \| Minimum
Subsequent Event [Line Items]
Fair value of shares affected	$ 2,400,000
2019 \| Maximum
Subsequent Event [Line Items]
Fair value of shares affected	$ 3,200,000

Share Based Compensation Arrangement By Share-based Payment, Award Initial Measurement Period

No definition available.

Name:	denn_ShareBasedCompensationArrangementByShareBasedPaymentAwardInitialMeasurementPeriod
Namespace Prefix:	denn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Share based Compensation Arrangement By Share-based Payment Award, Percent of Award Issued

+ References

No definition available.

+ Details

Name:	denn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentOfAwardIssued
Namespace Prefix:	denn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Share Based Compensation Arrangement By Share-based Payment Award, Percentage of Growth in Earnings

+ References

No definition available.

+ Details

Name:	denn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfGrowthInEarnings
Namespace Prefix:	denn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Share Based Compensation Arrangement By Share-based Payment Award, Performance Goal Exceeded, Pro-rated Percentage

+ References

No definition available.

+ Details

Name:	denn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceGoalExceededProRatedPercentage
Namespace Prefix:	denn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Share-Based Compensation Arrangement By Share-Based Payment Award, Plan Modification, Equity Instruments Other Than Options, Fair Value

+ References

No definition available.

+ Details

Name:	denn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationEquityInstrumentsOtherThanOptionsFairValue
Namespace Prefix:	denn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Share-Based Compensation Arrangement By Share Based Payment Award, Plan Modification, Equity Instruments Other Than Options Nonvested Weighted Average Grant Date Fair Value

+ References

No definition available.

+ Details

Name:	denn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
Namespace Prefix:	denn_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Share-Based Compensation Arrangement By Share-Based Payment Award, Plan Modification, Equity Instruments Other Than Options, Number Of Shares Affected By Modification

+ References

No definition available.

+ Details

Name:	denn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationEquityInstrumentsOtherThanOptionsNumberOfSharesAffectedByModification
Namespace Prefix:	denn_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Share based Compensation Arrangement By Share-based Payment Award, Plan Modification, Percentage Of Target

+ References

No definition available.

+ Details

Name:	denn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationPercentageOfTarget
Namespace Prefix:	denn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Plan Modification, Vesting Period

+ References

No definition available.

+ Details

Name:	denn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationVestingPeriod
Namespace Prefix:	denn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Share-based Compensation Arrangement By Share-based Payment Award, Relative Total Stockholder Return, Percentage

+ References

No definition available.

+ Details

Name:	denn_ShareBasedCompensationArrangementByShareBasedPaymentAwardRelativeTotalStockholderReturnPercentage
Namespace Prefix:	denn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_PerformanceSharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=denn_TwentyEighteenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_PlanNameAxis=denn_TwentyNineteenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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