Stock-Based Compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	73,120
Stock-based compensation expense	$ 1,405	$ 1,036	$ 1,199
Cost of revenues
Stock-based compensation
Stock-based compensation expense	144	148	154
Selling, general and administrative expense.
Stock-based compensation
Stock-based compensation expense	$ 1,261	$ 888	$ 1,045
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)	59,500
Granted (in shares)	9,900
Exercised (in shares)	(2,000)
Outstanding at the end of the period (in shares)	67,400	59,500
Exercisable at the end of the period (in shares)	67,400
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 58.36
Granted (in dollars per share)	77.89
Exercised (in dollars per share)	35.95
Outstanding at the end of the period (in dollars per share)	61.90	$ 58.36
Exercisable at the end of the period (in dollars per share)	$ 61.90
Aggregate Intrinsic Value
Outstanding at the beginning of the period	$ 1,035
Exercised	86
Outstanding at the end of the period	1,033	$ 1,035
Exercisable at the end of the period	$ 1,033
Additional disclosures
Weighted-average fair value of stock options granted during the year (in dollars per share)	$ 17.61	$ 13.12	$ 10.79
Weighted-average remaining contractual life for stock options	6 years 6 months 26 days	6 years 9 months 11 days	6 years 3 months 26 days
Total fair value of stock options vested during the year	$ 174	$ 130	$ 107
Total intrinsic value of stock options exercised during the year	$ 86	$ 150	$ 30
Weighted-average remaining contractual life of the outstanding and exercisable stock options	6 years 6 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
Expected life	3 years
Options | Minimum
Weighted-average assumptions used to estimate the fair value for the stock options
Risk-free interest rates (as a percent)	1.45%	0.92%	0.98%
Dividend yield (as a percent)	0.54%	0.67%	0.80%
Volatility factor (as a percent)	0.258%	0.264%	0.272%
Options | Maximum
Weighted-average assumptions used to estimate the fair value for the stock options
Risk-free interest rates (as a percent)	1.81%	1.49%	1.28%
Dividend yield (as a percent)	0.67%	1.00%	0.91%
Volatility factor (as a percent)	0.265%	0.275%	0.284%
Options | Weighted-average
Weighted-average assumptions used to estimate the fair value for the stock options
Risk-free interest rates (as a percent)	1.73%	1.37%	1.05%
Dividend yield (as a percent)	0.57%	0.74%	0.83%
Volatility factor (as a percent)	0.26%	0.266%	0.275%
Restricted stock
Restricted Stock
Non-vested at the beginning of the period (in shares)	17,697
Granted (in shares)	17,179
Vested (in shares)	(16,730)
Forfeited (in shares)	(482)
Non-vested at the end of the period (in shares)	17,665	17,697
Weighted-Average Grant-Date Fair Value
Non-vested at the beginning of the period (in dollars per share)	$ 64.01
Granted (in dollars per share)	77.44
Vested (in dollars per share)	73.32
Forfeited (in dollars per share)	65.97
Non-vested at the end of the period (in dollars per share)	$ 69.96	$ 64.01
Additional disclosures
Total fair value of restricted stock vested during the year	$ 1,232	$ 907	$ 1,099
Total compensation cost not yet recognized	$ 1,135
Weighted-average period for recognition of total compensation cost not yet recognized	1 year 1 month 2 days
Restricted stock | Minimum
Stock-based compensation
Vesting period	6 months
Restricted stock | Maximum
Stock-based compensation
Vesting period	3 years