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PROPERTY AND EQUIPMENT, NET - Headquarters location (Details) - USD ($)
$ in Millions
3 Months Ended
Jun. 30, 2020
Sep. 30, 2021
Dec. 31, 2020
Property and Equipment, Net      
Property held and used   $ 102.4 $ 82.6
Land   2.3  
Property and equipment   102.4 $ 82.6
Chicago Headquarters Location      
Property and Equipment, Net      
Property held and used   12.2  
Land   2.3  
Property and equipment   $ 9.9  
Chicago Headquarters Location | Options      
Property and Equipment, Net      
Impairment charge $ 8.1