Stock-Based Compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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	45,157
Stock-based compensation expense	$ 1,489	$ 1,405	$ 1,036
Cost of revenues
Stock-based compensation
Stock-based compensation expense	154	144	148
Selling, general and administrative expense.
Stock-based compensation
Stock-based compensation expense	$ 1,335	$ 1,261	$ 888
Maximum
Stock-based compensation
Number of shares of common stock that may be subject to outstanding awards granted under the 2001 Plan	741,413
Options
Stock-based compensation
Expiration period	10 years
Stock Options
Outstanding at the beginning of the period (in shares)	67,400
Granted (in shares)	9,900
Exercised (in shares)	(17,000)
Outstanding at the end of the period (in shares)	60,300	67,400
Exercisable at the end of the period (in shares)	60,300
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 61.90
Granted (in dollars per share)	71.61
Exercised (in dollars per share)	57.80
Outstanding at the end of the period (in dollars per share)	64.64	$ 61.90
Exercisable at the end of the period (in dollars per share)	$ 64.64
Aggregate Intrinsic Value
Outstanding at the beginning of the period	$ 1,033
Exercised	447
Outstanding at the end of the period	507	$ 1,033
Exercisable at the end of the period	$ 507
Additional disclosures
Weighted-average fair value of stock options granted during the year (in dollars per share)	$ 17.74	$ 17.61	$ 13.12
Weighted-average remaining contractual life for stock options	6 years 10 months 13 days	6 years 6 months 26 days	6 years 9 months 11 days
Total fair value of stock options vested during the year	$ 176	$ 174	$ 130
Total intrinsic value of stock options exercised during the year	$ 447	$ 86	$ 150
Weighted-average remaining contractual life of the outstanding and exercisable stock options	6 years 10 months 13 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)	2.51%	1.45%	0.92%
Dividend yield (as a percent)	0.73%	0.54%	0.67%
Volatility factor (as a percent)	0.256%	0.258%	0.264%
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)	2.80%	1.81%	1.49%
Dividend yield (as a percent)	0.76%	0.67%	1.00%
Volatility factor (as a percent)	0.249%	0.265%	0.275%
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)	2.59%	1.73%	1.37%
Dividend yield (as a percent)	0.75%	0.57%	0.74%
Volatility factor (as a percent)	0.25%	0.26%	0.266%
Restricted stock
Restricted Stock
Non-vested at the beginning of the period (in shares)	17,665
Granted (in shares)	18,063
Vested (in shares)	(17,732)
Forfeited (in shares)	(402)
Non-vested at the end of the period (in shares)	17,594	17,665
Weighted-Average Grant-Date Fair Value
Non-vested at the beginning of the period (in dollars per share)	$ 69.96
Granted (in dollars per share)	72.12
Vested (in dollars per share)	74.85
Forfeited (in dollars per share)	72.42
Non-vested at the end of the period (in dollars per share)	$ 71.36	$ 69.96
Additional disclosures
Total fair value of restricted stock vested during the year	$ 1,327	$ 1,232	$ 907
Total compensation cost not yet recognized	$ 1,258
Weighted-average period for recognition of total compensation cost not yet recognized	1 year 2 months 23 days
Restricted stock | Minimum
Stock-based compensation
Vesting period	6 months
Restricted stock | Maximum
Stock-based compensation
Vesting period	3 years