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Summary of Acquisitions (Parenthetical) (Detail)
$ in Thousands
9 Months Ended
Sep. 03, 2017
USD ($)
Property
Sep. 30, 2018
USD ($)
Property
Business Acquisition [Line Items]    
Number of units in real estate properties acquired | Property   1,843
Purchase Price | $   $ 195,448
Acquisition of Nine-Property Portfolio First Phase Acquisition    
Business Acquisition [Line Items]    
Number of units in real estate properties acquired | Property 2,352  
Purchase Price | $ $ 228,144