Segment Reporting (Details 1) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reconciliation of segment information to consolidated financial statements
Operating Income									$ 128,716	$ 118,456	$ 107,897
Unallocated corporate expenses									(50,247)	(36,571)	(36,516)
Total operating income									128,716	118,456	107,897
Interest, net (Note 6)	(2,225)	(2,684)	(2,086)	(2,604)	(2,582)	(2,256)	(2,194)	(2,063)	(9,599)	(9,095)	(6,341)
Loss from equity in joint ventures									(4,724)	(3,616)	(1,663)
Other, net									1,329	(851)	1,586
Income Before Provision For Income Taxes	21,249	30,235	31,518	32,720	18,198	28,229	31,332	27,135	115,722	104,894	101,479
Assets	985,478				901,118				985,478	901,118	811,431
Unallocated corporate assets	34,764				48,368				34,764	48,368	79,593
Consolidated assets	985,478				901,118				985,478	901,118	811,431
Unallocated corporate expenditures									1,842	893	2,229
Consolidated capital expenditures									83,159	83,166	73,748
Depreciation and amortization expenses									51,294	47,099	40,351
Unallocated corporate depreciation expenses									1,660	2,248	2,130
Consolidated depreciation and amortization expenses									51,294	47,099	40,351
Reportable Segment [Member]
Reconciliation of segment information to consolidated financial statements
Operating Income									178,963	155,027	144,413
Total operating income									178,963	155,027	144,413
Assets	950,714				852,750				950,714	852,750	731,838
Consolidated assets	950,714				852,750				950,714	852,750	731,838
Capital expenditures									81,317	82,273	81,919
Depreciation and amortization expenses									$ 49,634	$ 44,851	$ 38,221