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Income Taxes - Provision for Income Taxes and Effective Income Tax Rates (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Income Tax Disclosure [Abstract]        
Provision for income taxes $ 227 $ 371 $ 724 $ 539
Effective income tax rate 1.70% 90.70% 2.80% 65.60%