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Business Acquisitions (Details) - USD ($)
$ in Millions
1 Months Ended 9 Months Ended
Apr. 30, 2018
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Business Acquisition [Line Items]        
Acquisitions, net of cash acquired     $ (1,102)  
Ownership Percentage Acquired     25.00%  
Purchase of redeemable noncontrolling interest   $ 131 $ 0  
Goodwill   9,683   $ 9,678
Other acquisitions [Member]        
Business Acquisition [Line Items]        
Acquisitions, net of cash acquired   $ (105) (1,100)  
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets     1,100  
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities     476  
Goodwill     $ 522  
HomeServices [Member] | Real estate brokerage franchise business [Member]        
Business Acquisition [Line Items]        
Ownership Percentage Acquired 33.30%      
Purchase of redeemable noncontrolling interest $ 131