Stock Compensation (Narrative) (Details) (USD $)	1 Months Ended	3 Months Ended		6 Months Ended
	Jan. 31, 2011	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Weighted average grant date fair value of stock granted
Restricted stock units, Granted
Options and restricted stock units authorized to grant		872,000		872,000
Exercise of stock options, shares				24,600
Proceeds from the exercise of stock options				$ 1,050,000	$ 5,896,000
Number of Units, vested				24,230
1997 Plan [Member]
Number of shares covered under stock option and incentive plan		1,500,000		1,500,000
2003 Plan [Member]
Number of shares covered under stock option and incentive plan		1,500,000		1,500,000
Stock Options [Member]
Weighted average grant date fair value of options granted				$ 5.38	$ 6.08
Dividend yield				2.90%	3.30%
Expected volatility				13.90%	17.50%
Expected life (in years)				5	5
Risk-free interest rate				1.70%	2.40%
Stock option expense		126,000	147,000	262,000	241,000
Exercise of stock options, shares				24,600	181,036
Aggregate intrinsic value of the stock options exercised				457,000	3,700,000
Unamortized compensation expenses		1,400,000		1,400,000
Weighted average recognized period of unamortized compensation expenses (in years)				3.6
Restricted Stock [Member]
Weighted average grant date fair value of stock granted					$ 52.35
Restricted stock units, Granted				0
Restricted stock units expense		202,000	334,000	488,000	813,000
Number of Units, vested				24,230	31,797
Aggregate fair value of the shares vested				1,400,000	1,700,000
Unamortized compensation expenses		2,100,000		2,100,000
Weighted average recognized period of unamortized compensation expenses (in years)				3.2
Stock compensation, net, shares				15,367	20,015
Retirement Plan for Non-Employee Directors [Member]
Shares approved for issuance				70,000
Number of shares granted for each year served				1,000
Maximum number of shares issued upon retirement				5,000
Compensation expense under retirement plan		36,000	39,000	72,000	81,000
Unamortized compensation expenses		268,000	411,000	268,000	411,000
Shares issued to a director upon retirement					0
Aggregate fair value of shares issued	$ 290,000
Stock compensation, net, shares	5,000