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Related Party Transactions - Additional Information (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Related Party Transaction [Line Items]    
Purchase of non-controlling interests $ 308 $ 224
Purchase of treasury stock shares 114,133 21,181
Partnership And Trust [Member]    
Related Party Transaction [Line Items]    
Purchase of non-controlling interests $ 308 $ 224
Related Party [Member]    
Related Party Transaction [Line Items]    
Purchase of treasury stock shares 10,000 10,000