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Income Taxes - Additional Information (Detail)
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Income Tax Disclosure [Abstract]        
US tax rate 34.00% 34.00% 34.00% 34.00%
Effective tax rate 1.00% 2.00% 1.00% 2.00%