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Stockholders' Equity - Narrative (Details) - USD ($)
1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2018
Jul. 31, 2018
Sep. 30, 2019
Sep. 30, 2019
Dec. 31, 2017
Feb. 13, 2019
Dec. 14, 2018
Convertible Preferred Stock              
Class of Stock [Line Items]              
Preferred stock liquidation preference percentage     4.00% 4.00%      
2018 Share Repurchase Program              
Class of Stock [Line Items]              
Stock repurchase program authorized amount             $ 1,000,000,000
Shares repurchased and retired (in shares) 10,000,000     8,000,000      
Aggregate value of shares repurchased and retired $ 536,000,000     $ 464,000,000      
Shares repurchased, average price per share (in usd per share) $ 53.46     $ 59.47      
2019 Share Repurchase Program              
Class of Stock [Line Items]              
Stock repurchase program authorized amount           $ 1,500,000,000  
Shares repurchased and retired (in shares)     0 17,000,000      
Aggregate value of shares repurchased and retired     $ 0 $ 883,000,000      
Shares repurchased, average price per share (in usd per share)       $ 50.70      
Remaining value of shares available for repurchase     $ 617,000,000 $ 617,000,000      
Offered in Connection With Forward Sale Agreements              
Class of Stock [Line Items]              
Number of shares issued in transaction (in shares)   6,000,000     6,000,000    
Offered in Connection With Forward Sale Agreements | MORGAN STANLEY AND CO. LLC              
Class of Stock [Line Items]              
Number of shares issued in transaction (in shares)         3,000,000    
Offered Directly By The Company              
Class of Stock [Line Items]              
Net proceeds from forward settlement   $ 349,000,000