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Goodwill and Intangible assets - Estimated Amortization Expense of Intangible Assets (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]    
2018 $ 5,415  
2019 3,868  
2020 2,401  
2021 2,266  
2022 834  
Thereafter 2,267  
Total $ 17,051 $ 13,273