Note 12 - Related Party Transactions (Details Textual) $ in Millions	3 Months Ended
Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Chalet [Member]
Number of Operating Leases	5
Number of Capital Leases	1
Related Party Transaction Number of Owners That Are Non-Independent Board Members of the Entity	4
Related Party Transaction, Expenses from Transactions with Related Party	$ 0.3	$ 0.3
Isely Family Land Trust LLC [Member]
Number of Operating Leases	1
Related Party Transaction, Expenses from Transactions with Related Party	$ 0.1	0.1
FTVC, LLC [Member]
Number of Operating Leases	1
Related Party Transaction Number of Owners That Are Non-Independent Board Members of the Entity	4
FTVC, LLC [Member] \| Maximum [Member]
Related Party Transaction, Expenses from Transactions with Related Party	$ 0.1	$ 0.1

Represents the number of capital leases, leased by the entity.

No definition available.

Name:	ngvc_NumberOfCapitalLeases
Namespace Prefix:	ngvc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the number of operating leases, leased by the entity.

+ References

No definition available.

+ Details

Name:	ngvc_NumberOfOperatingLeases
Namespace Prefix:	ngvc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the number of owners of the related party entity that are non-independent board members of the reporting entity.

+ References

No definition available.

+ Details

Name:	ngvc_RelatedPartyTransactionNumberOfOwnersThatAreNonIndependentBoardMembersOfTheEntity
Namespace Prefix:	ngvc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

+ Details

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

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=ngvc_ChaletMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=ngvc_IselyFamilyLandTrustLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=ngvc_FTVCLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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