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Summary of Acquisitions (Parenthetical) (Detail)
$ in Thousands
12 Months Ended
Sep. 03, 2017
USD ($)
Property
Dec. 31, 2017
USD ($)
Property
Business Acquisition [Line Items]    
Units (unaudited) | Property   2,340
Purchase Price   $ 243,585
Defeasance costs related to properties acquired $ 3,624  
HPI Portfolio    
Business Acquisition [Line Items]    
Units (unaudited) | Property 2,353  
Purchase Price $ 228,144