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Income Taxes - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2017
Sep. 30, 2016
Income Tax Disclosure [Abstract]      
Federal income tax at statutory rate (as a percent)   35.00% 35.00%
Discrete benefits associated with disqualifying dispositions of incentive stock options $ 3.2 $ 17.1