Comprehensive Income (Details) (USD $) In Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010		Dec. 31, 2010
Comprehensive Income [Abstract]
Net income	$ 58,577		$ 52,770		$ 57,939		$ 46,659
Foreign currency translation adjustments	1,779		(1,247)		1,841		(1,638)
Unrealized gains on interest rate swaps	433	[1]	235	[1]	861	[1]	327	[1]
Comprehensive income	60,789		51,758		60,641		45,348
Accumulated Other Comprehensive Income (Loss) [Abstract]
Foreign currency translation	4,926				4,926				3,085
Unrealized gains (losses) on interest rate swaps	(1,172)				(1,172)				(2,033)
Total	3,754		915		3,754		915		1,052
Other Comprehensive Income (Loss) [Abstract]
Foreign currency translation	1,779		(1,247)		1,841		(1,638)
Unrealized gains (losses) on interest rate swaps	$ 433	[1]	$ 235	[1]	$ 861	[1]	$ 327	[1]

[1] Amounts are shown net of tax.