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Commitments and Contingencies - Additional Information (Detail) - USD ($)
9 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Commitments and Contingencies [Line Items]      
Guarantee of affiliate's obligations $ 25,000,000    
Interest-only loan      
Commitments and Contingencies [Line Items]      
Due from affiliate $ 6,000,000 $ 6,000,000 $ 6,000,000
Due date Dec. 31, 2019    
Interest-only loan | London Interbank Offered Rate(LIBOR)      
Commitments and Contingencies [Line Items]      
Basis spread on variable rate 1.75%