Stock Based Compensation (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) plan $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Stock-based compensation
Number of stock-based employee compensation plans | plan	4
Number of executive stock award plans | plan	3
Total fair value of share awards vested | $	$ 2,670,000	$ 3,350,000	$ 3,530,000
Shares Available for Grant
Balance at the beginning of the period (in shares)	664,502	675,059	676,444
Granted (in shares)	(147,576)	(74,336)	(74,981)
Forfeited (in shares)	49	1,241	3,135
Balance at the end of the period (in shares)	577,208	664,502	675,059
Stock award plans
Additional disclosures
Stock-based compensation expense | $	$ 3,290,000	$ 2,760,000	$ 3,550,000
Income tax benefit due to stock-based compensation expenses | $	770,000	$ 650,000	$ 860,000
Unrecognized stock-based compensation expense | $	$ 7,630,000
Weighted-average period for unrecognized compensation cost to recognize	3 years 5 months 8 days
Executive Incentive Plan
Shares Available for Grant
Shares authorized	60,233	62,538	70,461
Additional disclosures
Vesting period	5 years
1982 Restricted Stock Plan | Maximum
Additional disclosures
Vesting period	10 years
Strategic Deployment Incentive Plan
Additional disclosures
Vesting period	5 years
Stock options
Additional disclosures
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	0	0	0
Stock-based compensation expense | $	$ 0	$ 0	$ 0
Stock options | Maximum
Additional disclosures
Contractual life	10 years
Non-vested stock awards outstanding
Number of Shares
Stock awards outstanding at the beginning of the period (in shares)	218,991	253,819	295,926
Granted (in shares)	147,576	74,336	74,981
Vested (in shares)	(74,203)	(100,299)	(106,513)
Forfeited (in shares)	(870)	(8,865)	(10,575)
Stock awards outstanding at the end of the period (in shares)	291,494	218,991	253,819
Weighted-Average Grant-Date Fair Value
Stock awards outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 29.60	$ 28.59	$ 27.41
Granted (in dollars per share) | $ / shares	37.41	31.44	29.11
Vested (in dollars per share) | $ / shares	28.95	28.35	25.79
Forfeited (in dollars per share) | $ / shares	31.82	30.28	27.51
Stock awards outstanding at the end of the period (in dollars per share) | $ / shares	$ 33.71	29.60	28.59
Employee Stock Purchase Plan
Additional disclosures
Required period of service	2 years
Maximum base rate of employee basic pay to purchase any dollar amount of stock (as a percent)	25.00%
Maximum stock which can be purchased by employee per year | $	$ 25,000
Purchases price (in dollars per share) | $ / shares	$ 34.35	$ 43.81	$ 53.84
Percentage of the premium fair market value of common stock as on the offering date or the market value as on the purchase date (as a percent)	1.78%	0.14%
Percentage of the discount fair market value of common stock as on the offering date or the market value as on the purchase date (as a percent)			(0.09%)
ESPP offering through June 2021
Additional disclosures
Value of shares to be purchased | $	$ 304,635
Stock value of most recent offering (in dollars per share ) | $ / shares	$ 34.35