Selected Quarterly Financial Data - (Unaudtited) (Details) (USD $)	3 Months Ended				12 Months Ended			3 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Mar. 31, 2011 Quarter 1 [Member]	Mar. 31, 2010 Quarter 1 [Member]	Jun. 30, 2011 Quarter 2 [Member]	Jun. 30, 2010 Quarter 2 [Member]	Sep. 30, 2011 Quarter 3 [Member]	Sep. 30, 2010 Quarter 3 [Member]	Sep. 30, 2010 Quarter 3 [Member] Additional [Member]	Dec. 31, 2011 Quarter 4 [Member]	Dec. 31, 2010 Quarter 4 [Member]
Current Foreign Tax Expense (Benefit) For Open Tax Years		$ 400,000
Revenues					638,793,000	596,076,000	564,178,000	115,281,000	107,421,000	165,301,000	149,848,000	192,321,000	183,801,000		165,890,000	155,006,000
Operating income					171,863,000	160,762,000	148,073,000	25,679,000	23,837,000	45,122,000	43,607,000	62,400,000	54,877,000		38,662,000	38,441,000
Income before income taxes					158,057,000	158,211,000	150,634,000	21,923,000	24,646,000	42,114,000	42,024,000	56,966,000	55,026,000		37,054,000	36,515,000
Net income					110,396,000	107,441,000	98,250,000	15,730,000	15,793,000	27,578,000	27,011,000	42,302,000	40,494,000		24,786,000	24,143,000
Per basic share [Abstract]
Net income					$ 1.86	$ 1.80	$ 1.64	$ 0.26	$ 0.27	$ 0.46	$ 0.45	$ 0.71	$ 0.68		$ 0.42	$ 0.41
Per diluted share [Abstract]
Net income					$ 1.85	$ 1.80	$ 1.63	$ 0.26	$ 0.26	$ 0.46	$ 0.45	$ 0.71	$ 0.68		$ 0.42	$ 0.40
Termination Benefits Included In Operating results	3,600,000	400,000	300,000	100,000					400,000		(100,000)		300,000			1,200,000
Impairment of Long-Lived Assets to be Disposed of				1,800,000
Tax Adjustments, Settlements, and Unusual Provisions					1,400,000
Income Tax Reconciliation, Tax Credits, Foreign		1,700,000			2,800,000	1,600,000
Unrecognized Tax Benefits, Increases Resulting from Current Period Tax Positions		1,900,000			(173,000)	18,000	33,000
Deferred Tax Assets Out Of Period Adjustment													3,300,000
Unrecognized Tax Benefits, Increases Resulting from Current Period Tax Positions					2,062,000	2,563,000	667,000						1,600,000
Current Federal Tax Expense (Benefit)					$ 40,642,000	$ 48,494,000	$ 41,833,000							$ 1,600,000