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Income Taxes - Schedule of Differences Between Effective Rate and United States Federal Income Tax Statutory Rate (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Disclosure [Abstract]      
Tax at statutory rate 35.00% 35.00% 35.00%
State income tax, net of federal benefit 3.33% 2.94% 2.97%
Non-deductible expenses 0.49% 0.43% 0.34%
Change in accrual estimates relating to uncertain tax positions 0.02% (2.11%) (0.43%)
Change in valuation allowance   0.48% 0.29%
Other, net 0.16% 0.06% (0.41%)
Change in tax law (17.06%)    
Income tax provision 21.94% 36.80% 37.76%