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Income Taxes - Schedule of Income Tax Provisions (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Current:      
U.S. federal $ 21,272    
State and local 2,608 $ 2,015 $ 622
Total current provision 23,880 2,015 622
Deferred:      
U.S. federal 483 6,124  
State and local 1,322 42  
Total deferred provision 1,805 6,166  
Total tax provision $ 25,685 $ 8,181 $ 622