Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities:
Net income	$ 9,288	$ 7,943
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	1,610	1,477
Net amortization of premiums on securities	671	824
Provision for loan losses	2,949	2,781
Share-based compensation	302	239
Deferred income tax (benefit) expense	(2,691)	679
Proceeds from sale of loans held for sale	5,389	1,663
Originations of loans held for sale	(4,098)	(2,662)
Income on company owned life insurance	(450)	(445)
Net gain on sale of loans held for sale	(96)	(48)
Net gain on investment securities		(206)
Net (gain) loss on other assets	(3)	2
Decrease (increase) in other assets	7,587	(2,013)
Decrease in other liabilities	(3,351)	(2,937)
Net cash provided by operating activities	17,107	7,297
Cash flows from investing activities:
Purchases of available for sale securities		(22,544)
Purchases of held to maturity securities	(2,767)	(16,276)
Proceeds from principal payments, maturities and calls on available for sale securities	6,167	10,566
Proceeds from principal payments, maturities and calls on held to maturity securities	16,883	14,470
Proceeds from sales of securities available for sale		12,350
Net loan originations	(60,644)	(65,100)
Purchases of company owned life insurance, net of proceeds received	(7)	(7)
Proceeds from sales of other assets	24	27
Purchases of premises and equipment	(383)	(5,290)
Net cash used in investing activities	(40,727)	(71,804)
Cash flows from financing activities:
Net increase in deposits	169,835	174,437
Net decrease in short-term borrowings	(118,600)	(28,200)
Purchases of common stock for treasury	(113)	(148)
Proceeds from stock options exercised	76	243
Cash dividends paid to common and preferred shareholders	(3,859)	(3,403)
Net cash provided by financing activities	47,339	142,929
Net increase in cash and cash equivalents	23,719	78,422
Cash and cash equivalents, beginning of period	99,195	71,277
Cash and cash equivalents, end of period	122,914	149,699
Supplemental information:
Cash paid for interest	6,492	2,597
Cash paid for income taxes	1,571	4,500
Noncash investing and financing activities:
Real estate and other assets acquired in settlement of loans	370
Accrued and declared unpaid dividends	$ 4,181	3,408
Increase in net unsettled security purchases		$ 740

(Decrease) increase in net unsettled security purchases.

No definition available.

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

- Definition

The sum of the periodic adjustments of the differences between securities' face values and purchase prices that are charged against earnings. This is called accretion if the security was purchased at a discount and amortization if it was purchased at premium. As a noncash item, this element is an adjustment to net income when calculating cash provided by or used in operations using the indirect method.