Segment and Geographic Area Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Information [Line Items]
Number of Operating Regions									10
Revenues	$ 210,386	$ 217,210	$ 209,589	$ 191,911	$ 169,671	$ 156,492	$ 158,453	$ 145,579	$ 829,096	$ 630,195	$ 544,866
Income from operations	45,388	56,023	51,165	39,203	23,919	38,512	37,873	30,106	191,779	130,410	99,322
Depreciation and amortization									66,481	51,166	48,110
Interest expense									(38,617)	(23,147)	(26,902)
Interest income									(3,243)	(2,397)	(1,065)
Provision for income taxes									38,531	30,164	15,916
Expenditures for additions to property & equipment, net of grants from outside parties									(156,026)	(79,038)	(64,290)
North American & European Operations [Member]
Segment Reporting Information [Line Items]
Revenues									557,621	494,787	451,446
Income from operations									129,646	119,902	84,117
Depreciation and amortization									47,218	43,807	43,219
Interest expense									(23,171)	(21,856)	26,902
Interest income									(2,950)	(485)	94
Provision for income taxes									26,181	27,176	11,289
Expenditures for additions to property & equipment, net of grants from outside parties									(59,383)	(59,153)	(54,375)
Australian Operations [Member]
Segment Reporting Information [Line Items]
Revenues									271,475	135,408	93,420
Income from operations									62,133	10,508	15,205
Depreciation and amortization									19,263	7,359	4,891
Interest expense									(15,446)	(1,291)	0
Interest income									(293)	(1,912)	971
Provision for income taxes									12,350	2,988	4,627
Expenditures for additions to property & equipment, net of grants from outside parties									$ (96,643)	$ (19,885)	$ (9,915)