Supplemental Consolidated Cash Flow Information (Additional Information Related To Cash Flows) (Details) - USD ($) $ in Thousands	4 Months Ended
Jan. 19, 2020	Jan. 20, 2019
Supplemental Cash Flow Information [Abstract]
Decrease in obligations for treasury stock repurchases	$ 2,025	$ 14,362
Decrease in obligations for purchases of property and equipment	2,377	4,927
Increase (decrease) in dividends accrued or converted to stock equivalents	63	58
Decrease in finance lease obligations from the termination of equipment and building leases	$ 0	$ 7

The increase (decrease) during the reporting period in the amount of capital lease obligations that result in (decrease) increase in the related capital lease asset.

No definition available.

+ Details

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Namespace Prefix:	jack_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Increase (decrease) in dividends accrued or converted to stock equivalents, adjustment to dividends paid, financing activities

+ References

No definition available.

+ Details

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Namespace Prefix:	jack_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The increase (decrease) during the reporting period of obligations, not classified as trade payables, incurred and payable for the acquisition of merchandise, materials, supplies, and services pertaining to construction projects.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

+ Details

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Balance Type:	debit
Period Type:	duration

- Definition

The increase (decrease) during the reporting period in the liability under agreements to repurchase securities that were sold.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

+ Details

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Balance Type:	debit
Period Type:	duration

- References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration