Annual report pursuant to Section 13 and 15(d)

Goodwill and Other Intangible Assets - Change in Carrying Amount of Goodwill (Detail)

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Goodwill and Other Intangible Assets - Change in Carrying Amount of Goodwill (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Goodwill [Roll Forward]    
Beginning balance $ 259,938 $ 220,097
Goodwill acquired 491,079 39,890
Other 35 (49)
Ending balance $ 751,052 $ 259,938