Debt (Details Textual) (USD $)	9 Months Ended
	Sep. 30, 2012 banks	Dec. 31, 2011
Debt (Additional Textual) [Abstract]
Revolving credit agreement	$ 125,000,000
Number of other U.S. banks involved in revolving credit agreement	4
Duration of revolving credit agreement	5 years
Line of credit facility, Minimum maturity period	1 day
Line of credit facility, Maximum maturity period	180 days
Debt Outstanding	0
Letters of Credit outstanding	2,857,000
Unused Revolving credit	122,143,000
Unrestricted retained earnings	108,928,000	184,738,000
Maximum amount of dividends and treasury shares purchase under revolving credit agreement	$ 100,000,000	$ 30,000,000
Percentage of restricted group net income and cash proceeds of stock option exercises	100.00%	100.00%
Maximum [Member]
Debt (Textual) [Abstract]
Spread on LIBOR rate	1.65%
Spread on Prime rate	0.65%
Facility fee percentage	0.35%
Minimum [Member]
Debt (Textual) [Abstract]
Spread on LIBOR rate	0.95%
Spread on Prime rate	0.00%
Facility fee percentage	0.175%