Business Combinations - Results of Operations (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual	$ 28,200	$ 112,200	$ 63,800
Business Combination Pro Forma Information Income Loss Before Income Taxes Of Acquiree Since Acquisition Date Actual	1,500	(1,300)	900
Two Thousand Fourteen Acquisitions [Member]
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [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 (in dollars per share)		$ 1.51	$ 1.72
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Diluted		$ 1.49	$ 1.70
Weighted Average Basic Shares Outstanding, Pro Forma		140,944	141,033
Weighted average number of shares outstanding:
Pro Forma Weighted Average Shares Outstanding, Diluted		142,891	142,624
Two Thousand Thirteen Acquisitions [Member]
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [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 (in dollars per share)			$ 1.57	$ 1.39
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Diluted			$ 1.55	$ 1.36
Weighted Average Basic Shares Outstanding, Pro Forma			141,033	139,634
Weighted average number of shares outstanding:
Pro Forma Weighted Average Shares Outstanding, Diluted			142,624	142,010
Two Thousand Fifteen Acquisitions [Member]
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Total revenues	1,688,297	$ 1,630,992
Income before income taxes	411,497	356,426
Net income	$ 248,720	$ 217,053
Net income per share:
Basic (in dollars per share)	$ 1.76	$ 1.50
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Diluted	$ 1.73	$ 1.48
Weighted Average Basic Shares Outstanding, Pro Forma	137,810	140,944
Weighted average number of shares outstanding:
Pro Forma Weighted Average Shares Outstanding, Diluted	140,112	142,891