Results of Operations (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 2013 Acquisitions [Member]	Sep. 30, 2012 2013 Acquisitions [Member]	Sep. 30, 2013 2013 Acquisitions [Member]	Sep. 30, 2012 2013 Acquisitions [Member]	Sep. 30, 2012 2012 Acquisitions [Member]	Sep. 30, 2011 2012 Acquisitions [Member]	Sep. 30, 2012 2012 Acquisitions [Member]	Sep. 30, 2011 2012 Acquisitions [Member]
Business Acquisition, Pro Forma Information [Line Items]
Total revenues					$ 359,310	$ 333,375	$ 1,080,725	$ 978,275	$ 304,776	$ 292,599	$ 907,796	$ 866,115
Income before income taxes					95,455	87,038	292,743	250,428	81,998	81,785	239,286	238,144
Net income					$ 57,749	$ 52,771	$ 176,881	$ 150,317	$ 49,715	$ 49,699	$ 143,630	$ 144,021
Net income per share:
Basic					$ 0.40	$ 0.37	$ 1.22	$ 1.05	$ 0.35	$ 0.35	$ 1.00	$ 1.01
Diluted					$ 0.39	$ 0.36	$ 1.21	$ 1.03	$ 0.34	$ 0.34	$ 0.98	$ 1.00
Weighted average number of shares outstanding:
Basic	141,139	139,465	140,925	139,185	141,139	139,465	140,925	139,185	139,465	138,690	139,185	138,475
Diluted	142,789	142,097	142,476	141,769	142,789	142,097	142,476	141,769	142,097	140,443	141,769	140,120