PRO FORMA FINANCIAL INFORMATION (Details)
$ / shares in Units, $ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2017
$ / shares
Dec. 31, 2016
USD ($)
Sep. 30, 2016
$ / shares
Sep. 30, 2017
USD ($)
property
$ / shares
Sep. 30, 2016
USD ($)
$ / shares
Dec. 31, 2016
property
Consolidated results of operations on a pro forma basis            
Pro forma revenue | $       $ 416,145 $ 389,108  
Pro forma income | $       $ 105,736 $ 84,530  
Earnings per share from continuing operations:            
Basic - as reported (in dollars per share) $ 0.21   $ 0.13 $ 0.53 $ 0.32  
Diluted - as reported (in dollars per share) $ 0.21   $ 0.13 0.52 0.32  
Basic - as pro forma (in dollars per share)       0.58 0.45  
Diluted - as pro forma (in dollars per share)       $ 0.58 $ 0.45  
Self-storage facilities            
Self-storage facilities            
Number of self-storage facilities acquired | property       3   28
Aggregate purchase price | $   $ 403,600   $ 33,100