Retirement Benefit Plans (Tables)

12 Months Ended

Dec. 31, 2017

Compensation and Retirement Disclosure [Abstract]

Schedule of Defined Benefit Plans Disclosures

The following tables set forth changes in the plans’ benefit obligations (projected benefit obligation for pension benefits and accumulated benefit obligation for postretirement benefits) and changes in the plans’ assets and the funded status of the pension plans and other postretirement benefit plan at December 31:

Schedule of Defined Benefit Plan Disclosures
	Employees' Pension Plan			Select Executive Retirement Plan			Postretirement Benefit Plans
Defined Benefit Plan Funded Status
December 31, 2017
Fair Value of Plan Assets	$	53,571		$	—		$	—
Benefit Obligation	38,921			4,586			7,727
Funded Status of Plan	$	14,650		$	(4,586	)	$	(7,727	)
December 31, 2016
Fair Value of Plan Assets	$	50,220		$	—		$	—
Benefit Obligation	36,154			4,547			7,623
Funded Status of Plan	$	14,066		$	(4,547	)	$	(7,623	)
Change in Benefit Obligation
Benefit Obligation, at January 1, 2017	$	36,154		$	4,547		$	7,623
Service Cost	1,392			45			130
Interest Cost	1,682			209			339
Plan Participants' Contributions	—			—			492
Amendments	—			—			—
Actuarial (Gain) Loss	2,440			245			(14		)
Benefits Paid	(2,747		)	(460		)	(843		)
Benefit Obligation, at December 31, 2017	$	38,921		$	4,586		$	7,727

Schedule of Defined Benefit Plan Disclosures
	Employees' Pension Plan			Select Executive Retirement Plan			Postretirement Benefit Plans
Benefit Obligation, at January 1, 2016	$	35,982		$	4,784		$	7,701
Service Cost	1,400			40			147
Interest Cost	1,641			206			340
Plan Participants' Contributions	—			—			402
Amendments	—			—			—
Actuarial Gain	(738		)	(31		)	(327		)
Benefits Paid	(2,131		)	(452		)	(640		)
Benefit Obligation, at December 31, 2016	$	36,154		$	4,547		$	7,623
Change in Fair Value of Plan Assets
Fair Value of Plan Assets, at January 1, 2017	$	50,220		$	—		$	—
Actual Return on Plan Assets	6,098			—			—
Employer Contributions	—			460			351
Plan Participants' Contributions	—			—			492
Benefits Paid	(2,747		)	(460		)	(843		)
Fair Value of Plan Assets, at December 31, 2017	$	53,571		$	—		$	—
Fair Value of Plan Assets, at January 1, 2016	$	47,234		$	—		$	—
Actual Return on Plan Assets	5,117			—			—
Employer Contributions	—			452			238
Plan Participants' Contributions	—			—			402
Benefits Paid	(2,131		)	(452		)	(640		)
Fair Value of Plan Assets, at December 31, 2016	$	50,220		$	—		$	—
Accumulated Benefit Obligation at December 31, 2017	$	38,557		$	4,586		$	7,727
Amounts Recognized in the Consolidated Balance Sheets
December 31, 2017
Prepaid Pension Asset	$	14,650		$	—		—
Accrued Benefit Liability	—			(4,586		)	(7,727		)
Net Benefit Recognized	$	14,650		$	(4,586	)	$	(7,727	)
December 31, 2016
Prepaid Pension Asset	$	14,066		$	—		—
Accrued Benefit Liability	—			(4,547		)	(7,623		)
Net Benefit Recognized	$	14,066		$	(4,547	)	$	(7,623	)
Amounts Recognized in Other Comprehensive Income (Loss)
For the Year Ended December 31, 2017
Net Unamortized Gain Arising During the Period	$	(517	)	$	244		$	(14	)
Net Prior Service Cost Arising During the Period	—			—			—
Amortization of Net Loss	(306		)	(129		)	24
Amortization of Prior Service Credit (Cost)	57			(57		)	11
Total Other Comprehensive (Loss) for Pension and      Other Postretirement Benefit Plans	$	(766	)	$	58		$	21
For the Year Ended December 31, 2016
Net Unamortized Loss Arising During the Period	$	(2,657	)	$	(32	)	$	(328	)
Net Prior Service Cost Arising During the Period	—			—			—
Amortization of Net Loss	(591		)	(125		)	—
Amortization of Prior Service Credit (Cost)	57			(57		)	12
Total Other Comprehensive Income (Loss) for Pension and      Other Postretirement Benefit Plans	$	(3,191	)	$	(214	)	$	(316	)

Schedule of Defined Benefit Plan Disclosures
	Employees' Pension Plan			Select Executive Retirement Plan			Postretirement Benefit Plans
For the Year Ended December 31, 2015
Net Unamortized Loss Arising During the Period	$	472		$	(152	)	$	(1,715	)
Net Prior Service Cost Arising During the Period	277			91			—
Amortization of Net Loss	(601		)	(131		)	(114		)
Amortization of Prior Service (Cost) Credit	83			(58		)	31
Total Other Comprehensive (Loss) Income for Pension and      Other Postretirement Benefit Plans	$	231		$	(250	)	$	(1,798	)
Accumulated Other Comprehensive Income
December 31, 2017
Net Actuarial Loss	$	6,656		$	2,127		$	(228	)
Prior Service (Credit) Cost	264			489			433
Total Accumulated Other Comprehensive Income, Before Tax	$	6,920		$	2,616		$	205
December 31, 2016
Net Actuarial Loss	$	7,479		$	2,012		$	(238	)
Prior Service (Credit) Cost	207			546			422
Total Accumulated Other Comprehensive Income, Before Tax	$	7,686		$	2,558		$	184
Amounts that will be Amortized from Accumulated   Other Comprehensive Income the Next Year
Net Actuarial Loss	$	131		$	133		$	(2	)
Prior Service (Credit) Cost	$	(49	)	$	57		$	48
Net Periodic Benefit Cost
For the Year Ended December 31, 2017
Service Cost	$	1,392		$	45		$	130
Interest Cost	1,682			209			339
Expected Return on Plan Assets	(3,141		)	—			—
Amortization of Prior Service (Credit) Cost	(57		)	57			(11		)
Amortization of Net Loss	306			129			(24		)
Net Periodic Benefit Cost	$	182		$	440		$	434
For the Year Ended December 31, 2016
Service Cost	$	1,400		$	40		$	147
Interest Cost	1,641			206			340
Expected Return on Plan Assets	(3,198		)	—			—
Amortization of Prior Service (Credit) Cost	(57		)	57			(12		)
Amortization of Net Loss	591			125			—
Net Periodic Benefit Cost	$	377		$	428		$	475
For the Year Ended December 31, 2015
Service Cost	$	1,503		$	32		$	250
Interest Cost	1,545			211			394
Expected Return on Plan Assets	(3,311		)	—			—
Amortization of Prior Service (Credit) Cost	(83		)	58			(31		)
Amortization of Net Loss	601			131			114
Net Periodic Benefit Cost	$	255		$	432		$	727

Schedule of Defined Benefit Plan Disclosures
	Employees' Pension Plan		Select Executive Retirement Plan		Postretirement Benefit Plans
Weighted-Average Assumptions Used in   Calculating Benefit Obligation
December 31, 2017
Discount Rate	4.24	%	4.18	%	4.22	%
Rate of Compensation Increase	3.50	%	3.50	%	3.50	%
Interest Rate Credit for Determining   Projected Cash Balance Account	3.00	%	3.00	%
Interest Rate to Annuitize Cash       Balance Account	4.25	%	4.25	%
Interest Rate to Convert Annuities to Actuarially   Equivalent Lump Sum Amounts	4.25	%	4.25	%
December 31, 2016
Discount Rate	4.83	%	4.73	%	4.80	%
Rate of Compensation Increase	3.50	%	3.50	%	3.50	%
Interest Rate Credit for Determining   Projected Cash Balance Account	3.00	%	3.00	%
Interest Rate to Annuitize Cash       Balance Account	4.50	%	4.50	%
Interest Rate to Convert Annuities to Actuarially   Equivalent Lump Sum Amounts	4.50	%	4.50	%
Weighted-Average Assumptions Used in   Calculating Net Periodic Benefit Cost
December 31, 2017
Discount Rate	4.83	%	4.73	%	4.80	%
Expected Long-Term Return on Plan Assets	6.50	%
Rate of Compensation Increase	3.50	%	3.50	%	3.50	%
Interest Rate Credit for Determining       Projected Cash Balance Account	3.00	%	3.00	%
Interest Rate to Annuitize Cash       Balance Account	4.50	%	4.50	%
Interest Rate to Convert Annuities to Actuarially   Equivalent Lump Sum Amounts	4.50	%	4.50	%
December 31, 2016
Discount Rate	4.73	%	4.61	%	4.69	%
Expected Long-Term Return on Plan Assets	7.00	%
Rate of Compensation Increase	3.50	%	3.50	%	3.50	%
Interest Rate Credit for Determining       Projected Cash Balance Account	3.03	%	3.03	%
Interest Rate to Annuitize Cash       Balance Account	5.00	%	5.00	%
Interest Rate to Convert Annuities to Actuarially   Equivalent Lump Sum Amounts	5.00	%	5.00	%
December 31, 2015
Discount Rate	4.31	%	4.26	%	4.31	%
Expected Long-Term Return on Plan Assets	7.50	%
Rate of Compensation Increase	3.50	%	3.50	%	3.50	%
Interest Rate Credit for Determining       Projected Cash Balance Account	3.04	%
Interest Rate to Annuitize Cash       Balance Account	4.75	%
Interest Rate to Convert Annuities to Actuarially   Equivalent Lump Sum Amounts	4.75	%	4.75	%

Schedule of Defined Benefit Plan Disclosures Information about Defined Benefit Plan Assets - Employees' Pension Plan
Fair Value Measurements Using:
Asset Category	Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Total		Percent of Total		Target Allocation Minimum		Target Allocation Maximum
December 31, 2017
Cash	$	22	$	—	$	—	$	22	—	%	—	%	15.0	%
Interest-Bearing Money Market Fund	2,682		—		—		2,682		5.0	%	—	%	15.0	%
Arrow Common Stock1	5,657		—		—		5,657		10.6	%	—	%	10.0	%
North Country Funds - Equity 2	19,680		—		—		19,680		36.7	%
Other Mutual Funds - Equity	15,168		—		—		15,168		28.3	%
Total Equity Funds	34,848		—		—		34,848		65.0	%	55.0	%	85.0	%
North Country Funds - Fixed income 2	8,388		—		—		8,388		15.7	%
Other Mutual Funds - Fixed Income	1,974		—		—		1,974		3.7	%
Total Fixed Income Funds	10,362		—		—		10,362		19.4	%	10.0	%	30.0	%
Total	$	53,571	$	—	$	—	$	53,571	100.0	%
December 31, 2016
Cash	$	40	$	—	$	—	$	40	0.1	%	—	%	15.0	%
Interest-Bearing Money Market Fund	3,080		—		—		3,080		6.1	%	—	%	15.0	%
Arrow Common Stock1	6,592		—		—		6,592		13.1	%	—	%	10.0	%
North Country Funds - Equity 2	18,640		—		—		18,640		37.2	%
Other Mutual Funds - Equity	13,560		—		—		13,560		27.0	%
Total Equity Funds	32,200		—		—		32,200		64.2	%	55.0	%	85.0	%
North Country Funds - Fixed income 2	7,332		—		—		7,332		14.6	%
Other Mutual Funds - Fixed Income	976		—		—		976		1.9	%
Total Fixed Income Funds	8,308		—		—		8,308		16.5	%	10.0	%	30.0	%
Total	$	50,220	$	—	$	—	$	50,220	100.0	%

¹ Acquisition of Arrow Financial Corporation common stock was under 10% of the total fair value of the employee's pension plan assets at the time of acquisition.

² The North Country Funds - Equity and the North Country Funds - Fixed Income are publicly traded mutual funds advised by Arrow's subsidiary, North Country Investment Advisers, Inc.

Schedule of Defined Benefit Plan Disclosures
	Employees' Pension Plan		Select Executive Retirement Plan		Postretirement Benefit Plans
Expected Future Benefit Payments
2018	$	3,434	$	439	$	524
2019	2,345		426		543
2020	2,885		411		540
2021	2,456		396		581
2022	2,589		380		574
2022 - 2026	14,107		1,752		2,917
Estimated Contributions During 2018	$	—	$	439	$	524
Assumed Health Care Cost Trend Rates
December 31, 2017
Health Care Cost Trend   Rate Assumed for Next Year					7.25		%
Rate to which the Cost Trend   Rate is Assumed to Decline   (the Ultimate Trend Rate)					3.89		%
Year that the Rate Reaches    the Ultimate Trend Rate					2075
December 31, 2016
Health Care Cost Trend   Rate Assumed for Next Year					7.50		%
Rate to which the Cost Trend   Rate is Assumed to Decline   (the Ultimate Trend Rate)					3.89		%
Year that the Rate Reaches    the Ultimate Trend Rate					2075
Effect of a One-Percentage Point Change in Assumed   Health Care Cost Trend Rates
Effect of a One Percentage Point Increase on   Service and Interest Cost Components					$	44
Effect of a One Percentage Point Decrease on   Service and Interest Cost Components					(37		)
Effect of a One Percentage Point Increase on   Accumulated Postretirement Benefit Obligation					516
Effect of a One Percentage Point Decrease on   Accumulated Postretirement Benefit Obligation					(445		)