Goodwill and Other Intangible Assets - Schedule of Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Balance at beginning of period	$ 655,018	$ 584,634
Acquisitions	173,033	111,347
Divestitures		(40,963)
Balance at end of period	$ 828,051	$ 655,018