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Subsequent Events (Details) - USD ($)
$ in Millions
3 Months Ended
Dec. 31, 2019
Oct. 04, 2019
Sep. 30, 2019
Mar. 31, 2019
Subsequent Event [Line Items]        
Assets held for sale     $ 49 $ 23
Subsequent Event        
Subsequent Event [Line Items]        
Gain on sale of assets $ 30      
Subsequent Event | Herndon, Virginia        
Subsequent Event [Line Items]        
Assets held for sale   $ 23