Stock-Based Compensation (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Stock-based compensation
Number of shares of common stock that may be subject to outstanding awards granted under the 2001 Plan	650,000
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	113,318
Stock-based compensation expense	$ 1,082	$ 848	$ 737
Cost of revenues
Stock-based compensation
Stock-based compensation expense	188	158	156
Selling, general and administrative expense
Stock-based compensation
Stock-based compensation expense	894	690	581
Stock options
Stock-based compensation
Expiration period	10 years
Stock Options
Outstanding at the beginning of the period (in shares)	55,500
Granted (in shares)	9,900
Exercised (in shares)	(10,000)
Outstanding at the end of the period (in shares)	55,400	55,500
Exercisable at the end of the period (in shares)	55,400
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 30.92
Granted (in dollars per share)	$ 47.84
Exercised (in dollars per share)	$ 7.48
Outstanding at the end of the period (in dollars per share)	$ 38.13	$ 30.92
Exercisable at the end of the period (in dollars per share)	$ 38.13
Aggregate Intrinsic Value
Outstanding at the beginning of the period	1,582
Exercised	(559)	(606)	(1,107)
Outstanding at the end of the period	603	1,582
Exercisable at the end of the period	603
Additional disclosures
Weighted-average fair value of stock options granted during the year (in dollars per share)	$ 9.51	$ 13.99	$ 12.40
Weighted-average remaining contractual life for stock options	6 years 2 months 16 days	5 years 7 months 20 days	6 years 11 days
Total fair value of stock options vested during the year	94	133	118
Total intrinsic value of stock options exercised during the year	559	606	1,107
Weighted-average remaining contractual life of the outstanding and exercisable stock options	6 years 2 months 16 days
Weighted-average assumptions used to estimate the fair value for the stock options
Dividend yield (as a percent)	0.00%
Expected life	3 years
Stock options | Minimum
Stock-based compensation
Vesting period	0 years
Weighted-average assumptions used to estimate the fair value for the stock options
Risk-free interest rates (as a percent)	0.36%	0.39%	0.99%
Volatility factor (as a percent)	27.80%	35.10%	42.00%
Stock options | Maximum
Stock-based compensation
Vesting period	3 years
Weighted-average assumptions used to estimate the fair value for the stock options
Risk-free interest rates (as a percent)	0.51%	1.21%	1.81%
Volatility factor (as a percent)	28.80%	41.10%	42.30%
Stock options | Weighted-average
Weighted-average assumptions used to estimate the fair value for the stock options
Risk-free interest rates (as a percent)	0.40%	0.56%	1.16%
Volatility factor (as a percent)	28.60%	36.40%	42.20%
Restricted stock
Restricted Stock
Non-vested at the beginning of the period (in shares)	24,223
Granted (in shares)	17,235
Vested (in shares)	(18,898)
Forfeited (in shares)	(182)
Non-vested at the end of the period (in shares)	22,378	24,223
Weighted-Average Grant-Date Fair Value
Non-vested at the beginning of the period (in dollars per share)	$ 41.49
Granted (in dollars per share)	$ 53.00
Vested (in dollars per share)	$ 49.92
Forfeited (in dollars per share)	$ 53.29
Non-vested at the end of the period (in dollars per share)	$ 44.59	$ 41.49
Additional disclosures
Total fair value of restricted stock vested during the year	943	715	649
Total compensation cost not yet recognized	$ 769
Weighted-average period for recognition of total compensation cost not yet recognized	1 year 2 months 1 day
Restricted stock | Minimum
Stock-based compensation
Vesting period	6 months
Restricted stock | Maximum
Stock-based compensation
Vesting period	5 years