Stock-Based Compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock-based compensation
Maximum number of awards that may be received by an individual in any one calendar year (in shares)	100,000
Number of shares of common stock remaining available for future grants of stock options, restricted stock or other forms of stock-based compensation under the 2001 Plan	120,677
Stock-based compensation expense	$ 1,915	$ 1,524	$ 1,489
Cost of revenues
Stock-based compensation
Stock-based compensation expense	312	170	154
Selling, general and administrative expense.
Stock-based compensation
Stock-based compensation expense	$ 1,603	$ 1,354	$ 1,335
Maximum
Stock-based compensation
Number of shares of common stock that may be subject to outstanding awards granted under the 2001 Plan	874,589
Options
Stock-based compensation
Expiration period	10 years
Stock Options
Outstanding at the beginning of the period (in shares)	68,200
Granted (in shares)	9,900
Exercised (in shares)	(32,000)
Outstanding at the end of the period (in shares)	46,100	68,200
Exercisable at the end of the period (in shares)	46,100
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 68.81
Granted (in dollars per share)	105.28
Exercised (in dollars per share)	66.88
Outstanding at the end of the period (in dollars per share)	77.98	$ 68.81
Exercisable at the end of the period (in dollars per share)	$ 77.98
Aggregate Intrinsic Value
Outstanding at the beginning of the period	$ 1,466
Granted	86
Exercised	1,128
Outstanding at the end of the period	1,661	$ 1,466
Exercisable at the end of the period	$ 1,661
Additional disclosures
Weighted-average fair value of stock options granted during the year (in dollars per share)	$ 31.30	$ 20.47	$ 17.74
Weighted-average remaining contractual life for stock options	6 years 8 months 26 days	6 years 7 months 20 days	6 years 10 months 13 days
Total fair value of stock options vested during the year	$ 310	$ 203	$ 176
Total intrinsic value of stock options exercised during the year	$ 1,128	$ 57	$ 447
Weighted-average remaining contractual life of the outstanding and exercisable stock options	6 years 8 months 26 days
Non-vested stock options	0
Weighted-average assumptions used to estimate the fair value for the stock options
Time period used to calculate weighted averages for fair value assumptions	3 years
Options | Minimum
Weighted-average assumptions used to estimate the fair value for the stock options
Risk-free interest rates (as a percent)	0.37%	1.69%	2.51%
Dividend yield (as a percent)	0.56%	0.60%	0.73%
Volatility factor (as a percent)	0.346%	0.244%	0.249%
Expected life	3 years
Options | Maximum
Weighted-average assumptions used to estimate the fair value for the stock options
Risk-free interest rates (as a percent)	0.53%	2.33%	2.80%
Dividend yield (as a percent)	0.80%	0.67%	0.76%
Volatility factor (as a percent)	0.356%	0.247%	0.256%
Expected life	5 years
Options | Weighted-average
Weighted-average assumptions used to estimate the fair value for the stock options
Risk-free interest rates (as a percent)	0.42%	1.89%	2.59%
Dividend yield (as a percent)	0.64%	0.62%	0.75%
Volatility factor (as a percent)	0.354%	0.245%	0.25%
Restricted stock
Restricted Stock
Non-vested at the beginning of the period (in shares)	18,470
Granted (in shares)	18,823
Vested (in shares)	(18,618)
Forfeited (in shares)	(154)
Non-vested at the end of the period (in shares)	18,521	18,470
Weighted-Average Grant-Date Fair Value
Non-vested at the beginning of the period (in dollars per share)	$ 84.26
Granted (in dollars per share)	103.99
Vested (in dollars per share)	86.57
Forfeited (in dollars per share)	81.93
Non-vested at the end of the period (in dollars per share)	$ 103.10	$ 84.26
Additional disclosures
Total fair value of restricted stock vested during the year	$ 1,612	$ 1,303	$ 1,327
Total compensation cost not yet recognized	$ 1,699
Weighted-average period for recognition of total compensation cost not yet recognized	1 year 25 days
Restricted stock | Minimum
Stock-based compensation
Vesting period	6 months
Restricted stock | Maximum
Stock-based compensation
Vesting period	3 years