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Basis of Presentation (Narrative) (Details)	Sep. 30, 2019 item
Bank [Member]
Number of subsidiaries	2

X

- Definition

Number Of Subsidiaries

+ References

No definition available.

+ Details

Name:	flic_NumberOfSubsidiaries
Namespace Prefix:	flic_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	srt_OwnershipAxis=flic_FirstNationalBankOfLongIslandMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: