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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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Stock-Based Compensation - Stock Options, Stock Awards, and Stock in Lieu of Directors' Fees (Details) $ / shares in Units, $ in Thousands	1 Months Ended			12 Months Ended
	Jan. 31, 2021 shares	Jan. 31, 2020 shares	Jan. 31, 2019 $ / shares shares	Dec. 31, 2020 USD ($) plan $ / shares shares	Dec. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of active individual stock incentive plans \| plan				1
Stock-based compensation expense				$ 1,905	$ 1,872	$ 1,848
Tax benefit recognized related to stock-based compensation expense				535	526	519
Tax expense (benefit) from stock compensation				$ 25	(137)	(302)
Price as a percentage of fair market value that stock options may not be granted below				100.00%
Percentage of ownership of common stock by participants that if greater causes greater grant price				10.00%
Price as a percentage of fair market value that stock options may not be granted below for anyone who owns more than 10% common stock				110.00%
Expiration period of options granted for those who own more than 10% common stock				5 years
Vested and exercisable options in-the-money (in shares) \| shares				105,866
Net proceeds from exercise of stock options				$ 21	185	308
Total intrinsic value of options exercised				8	659	452
Cash paid by the Company to net settle employee taxes on restricted stock and options				233	402	559
Fair value of options vested				216	187	236
Fair value of restricted stock awards vested				1,300	1,200	1,700
Fair market share price (in usd per share) \| $ / shares			$ 33.50
Non-Employee Director
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Tax benefit recognized related to stock-based compensation expense				80	71	70
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense				190	190	200
Tax benefit recognized related to stock-based compensation expense				$ 53	$ 54	$ 56
Term in years				10 years	10 years	10 years
Cash paid by the Company to net settle employee taxes on restricted stock and options				$ 0	$ 157	$ 39
Unrecognized stock-based compensation expense				$ 324
Remaining vesting period unrecognized compensation expense is to be recognized				2 years 6 months
Restricted Stock and Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense				$ 1,400	1,400	1,400
Tax benefit recognized related to stock-based compensation expense				402	401	393
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cash paid by the Company to net settle employee taxes on restricted stock and options				233	245	520
Unrecognized stock-based compensation expense				$ 1,900
Remaining vesting period unrecognized compensation expense is to be recognized				2 years 3 months 18 days
Restricted Stock \| Employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period				4 years
Restricted Stock \| Non-Employee Director
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period				2 years
Common stock in Lieu of Cash \| Non-Employee Director
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense				$ 286	$ 253	$ 250
Number of shares issued in lieu of cash to directors (in shares) \| shares		8,346	7,470
Common stock in Lieu of Cash \| Non-Employee Director \| Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares issued in lieu of cash to directors (in shares) \| shares	11,532
Vesting, Year Two \| Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted, vesting percentage				50.00%
Vesting, Year Four \| Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted, vesting percentage				50.00%
2016 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares remained available for future grants (in shares) \| shares				183,561
Quarterly Average
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair market share price (in usd per share) \| $ / shares				$ 24.77	$ 30.35