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Business Combinations - Results of Operations (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Weighted average number of shares outstanding:
Basic	140,944	141,033	139,364
Diluted	142,891	142,624	142,010
2014 Acquisitions [Member]
Business Acquisition, Pro Forma Information [Line Items]
Total revenues	1,630,162	1,520,858
Income before income taxes	358,229	409,522
Net income	218,150	248,628
Net income per share:
Basic	1.51	1.72
Diluted	1.49	1.70
Weighted average number of shares outstanding:
Basic	140,944	141,033
Diluted	142,891	142,624
2013 Acquisitions [Member]
Business Acquisition, Pro Forma Information [Line Items]
Total revenues		1,439,918	1,329,262
Income before income taxes		373,175	329,291
Net income		226,562	198,826
Net income per share:
Basic		1.57	1.39
Diluted		1.55	1.36
Weighted average number of shares outstanding:
Basic		141,033	139,634
Diluted		142,624	142,010
2012 Acquisitions [Member]
Business Acquisition, Pro Forma Information [Line Items]
Total revenues			1,230,408	1,163,341
Income before income taxes			315,051	313,706
Net income			190,228	$ 190,174
Net income per share:
Basic			1.33	$ 1.33
Diluted			1.30	$ 1.31
Weighted average number of shares outstanding:
Basic			139,364	138,582
Diluted			142,010	140,264