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Supplementary Balance Sheet Information (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 24, 2016
Supplementary Balance Sheet Information [Abstract]
Employee insurance expense	$ 37.7	$ 33.1	$ 34.5

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- References

No definition available.

+ Details

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Namespace Prefix:	imkta_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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- Definition

The expense in the period incurred with respect to protection provided by insurance entities against risks other than risks associated with production (which are allocated to cost of sales).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.4)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

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