Foreign Currencies, Foreign Currency Translation and Comprehensive Income (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Functional Currencies Foreign Currency Translation And Comprehensive Income Loss [Abstract]
Foreign exchange transaction gains/(losses)	$ 1,000,000	$ (1,000,000)	$ 0	$ (1,000,000)
Total Comprehensive Income
Net income	18,723,000	17,550,000	38,987,000	33,230,000
Translation adjustment	1,507,000	(2,833,000)	6,231,000	(10,913,000)
Unrealized gain/(loss) on defined benefit plan, net of tax	(2,183,000)	(3,177,000)	5,129,000	(5,870,000)
Comprehensive income	18,047,000	11,540,000	50,347,000	16,447,000
Less: Comprehensive income attributable to noncontrolling interest	742,000	903,000	1,322,000	1,625,000
Total comprehensive income attributable to common stockholders	$ 17,305,000	$ 10,637,000	$ 49,025,000	$ 14,822,000