Results of Operations (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 2011 Acquisitions	Dec. 31, 2010 2011 Acquisitions	Dec. 31, 2010 2010 Acquisitions	Dec. 31, 2009 2010 Acquisitions	Dec. 31, 2012 2012 Acquisitions	Dec. 31, 2011 2012 Acquisitions
Business Acquisition, Pro Forma Information [Line Items]
Total revenues				$ 1,058,142	$ 1,059,857	$ 1,015,043	$ 1,035,286	$ 1,230,408	$ 1,163,341
Income before income taxes				283,404	291,944	278,635	274,908	315,051	313,706
Net income				$ 171,805	$ 177,464	$ 169,373	$ 165,420	$ 190,228	$ 190,174
Net income per share:
Basic				$ 1.20	$ 1.25	$ 1.19	$ 1.17	$ 1.33	$ 1.33
Diluted				$ 1.19	$ 1.23	$ 1.18	$ 1.16	$ 1.30	$ 1.31
Weighted average number of shares outstanding:
Basic	139,364	138,582	137,924	138,582	137,924	137,924	137,173	139,364	138,582
Diluted	142,010	140,264	139,318	140,264	139,318	139,318	137,507	142,010	140,264