Stockholders' Equity and Stock-based Compensation Arrangements (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 item	Jun. 30, 2012
Compensation cost included in:
Basic income per share (in dollars per share)	$ 0.01	$ 0.01	$ 0.01	$ 0.01
Diluted income per share (in dollars per share)	$ 0.01	$ 0.01	$ 0.01	$ 0.01
Stock-based compensation expense	$ 92,000	$ 100,000	$ 188,000	$ 211,000
Income tax benefit	18,000	21,000	39,000	47,000
Number of employee groups			2
Stock options
Number of Options
Outstanding at the beginning of the period (in shares)			641,050
Exercised (in shares)	(18,500)		(18,500)
Outstanding at the end of the period (in shares)	622,550		622,550
Exercisable at the end of the period (in shares)	582,550		582,550
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)			$ 22.12
Exercised (in dollars per share)			$ 17.99
Outstanding at the end of the period (in dollars per share)	$ 22.24		$ 22.24
Exercisable at the end of the period (in dollars per share)	$ 23.00		$ 23.00
Weighted-Average Remaining Contractual Life (in years)
Outstanding at the end of the period			4 years 7 months 6 days
Exercisable at the end of the period			4 years 4 months 24 days
Aggregate Intrinsic Value
Outstanding at the end of the period	5,011,000		5,011,000
Exercisable at the end of the period	4,261,000		4,261,000
Total intrinsic value of options exercised	200,000		200,000
Unrecognized compensation cost related to nonvested share-based compensation arrangements	100,000		100,000
Weighted-average period for compensation to be recognized			1 year
Cash received from option exercises under the stock plans	300,000		300,000
Employee Stock Purchase Plan
Fair value assumptions
Risk free interest rate (as a percent)	0.20%	0.20%	0.20%	0.20%
Weighted-average expected life of option feature	1 year	1 year	1 year	1 year
Expected volatility of underlying stock (as a percent)	28.70%	28.60%	28.70%	28.60%
Weighted-average fair value of option feature (in dollars per share)	$ 5.98	$ 7.19	$ 5.98	$ 7.19
Number of Options
Outstanding at the beginning of the period (in shares)			12,641
Shares subscribed			15,140
Forfeited (in shares)			(1,310)
Outstanding at the end of the period (in shares)	26,471		26,471
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)			$ 20.56
Shares subscribed (in dollars per share)			$ 20.56
Forfeited (in dollars per share)			$ 20.56
Outstanding at the end of the period (in dollars per share)	$ 20.56		$ 20.56
Remaining Contract Life (in years)
Outstanding at the end of the period			1 month 6 days
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 249,000		$ 249,000