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Goodwill and Intangible Assets - 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, January 1 $ 76,941 $ 49,600
Goodwill acquired 42,939 27,341
Balance, December 31 $ 119,880 $ 76,941