XML 48 R36.htm IDEA: XBRL DOCUMENT v3.10.0.1

Basis of Presentation And Summary Of Significant Accounting Policies (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($) State
Accounting Policies [Abstract]
Number of states in which services are provided | State	45
Reclassification from AOCI to retained earnings | $	$ 2.8

X

- Definition

Number of States in Which Entity Provides Services

+ References

No definition available.

+ Details

Name:	fisi_NumberOfStatesInWhichEntityProvidesServices
Namespace Prefix:	fisi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of increase (decrease) in accumulated other comprehensive income (AOCI) for reclassification to retained earnings of tax effect from remeasurement of deferred tax pursuant to Tax Cuts and Jobs Act.

+ References

No definition available.

+ Details

Name:	fisi_TaxCutsAndJobsActOf2017ReclassificationFromAociToRetainedEarningsTaxEffect
Namespace Prefix:	fisi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- References

No definition available.

+ Details

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