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EQUITY (Common Stock Offering) (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 20, 2019
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Class of Stock [Line Items]        
Proceeds from sale of shares $ 207.3      
Fees related to stock issuance $ 0.6      
AOG Units (in units)   231,883,861 218,613,369  
AOG        
Class of Stock [Line Items]        
Direct Ownership Interest   100.00% 100.00%  
Ares Owners Holdings, L.P.        
Class of Stock [Line Items]        
AOG Units (in units)   116,641,833 117,019,274  
Daily Average Ownership   51.98% 53.99% 55.52%
Ares Owners Holdings, L.P. | AOG        
Class of Stock [Line Items]        
Direct Ownership Interest   50.30% 53.53%  
Affiliate of Alleghany Corporation        
Class of Stock [Line Items]        
AOG Units (in units)   0 0  
Daily Average Ownership   0.00% 1.82% 5.89%
Affiliate of Alleghany Corporation | AOG        
Class of Stock [Line Items]        
Direct Ownership Interest   0.00% 0.00%  
AOG        
Class of Stock [Line Items]        
AOG Units (in units)   115,242,028 101,594,095  
Daily Average Ownership   48.02% 44.19% 38.59%
AOG | AOG        
Class of Stock [Line Items]        
Direct Ownership Interest   49.70% 46.47%  
Class A common stock | Underwritten        
Class of Stock [Line Items]        
Number of shares sold (in shares) 7,000,000