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Goodwill and Other Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]    
Balance, beginning of period $ 116,564 $ 89,533
Addition due to acqusitions 549,975 27,031
Balance, end of period $ 666,539 $ 116,564