Goodwill and Other Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Balance as of beginning of year	$ 421,563	$ 131,014
Goodwill from businesses acquired	1,563	293,390
Currency translation effect on foreign goodwill balances	10,248	(2,841)
Balance as of end of year	$ 433,374	$ 421,563