Business Combinations - Results of Operations (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual	$ 7,800	$ 34,200	$ 28,200
Business Combination Pro Forma Information Income Loss Before Income Taxes Of Acquiree Since Acquisition Date Actual	2,400	4,300	1,500
Two Thousand Seventeen Acquisition [Member] [Domain]
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Total revenues	1,891,701	1,784,776
Income before income taxes	453,397	429,490
Net income	$ 401,908	$ 261,133
Net income per share:
Basic (in dollars per share)	$ 2.88	$ 1.87
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Diluted	$ 2.83	$ 1.85
Weighted Average Basic Shares Outstanding, Pro Forma	136,290	136,139
Weighted average number of shares outstanding:
Pro Forma Weighted Average Shares Outstanding, Diluted	138,793	137,804
Two Thousand Fifteen Acquisitions [Member]
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Total revenues			1,688,297
Income before income taxes			411,497
Net income			$ 248,720
Net income per share:
Basic (in dollars per share)			$ 1.76
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Diluted			$ 1.73
Weighted Average Basic Shares Outstanding, Pro Forma			137,810
Weighted average number of shares outstanding:
Pro Forma Weighted Average Shares Outstanding, Diluted			140,112