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Stockholders' Equity (Narrative) (Details) - USD ($)
1 Months Ended 6 Months Ended
Feb. 07, 2018
Dec. 31, 2017
Dec. 31, 2016
Class A Common Stock      
Stock Repurchase Program [Abstract]      
Stock repurchase program, remaining authorized amount   $ 3,100,000,000  
Total number of shares repurchased   0  
Dividends [Abstract]      
Cash dividend declared per share   $ 0.18 $ 0.18
Class A Common Stock | Subsequent Event      
Dividends [Abstract]      
Cash dividend declared per share $ 0.18    
Cash dividend payable date Apr. 18, 2018    
Cash dividend record date Mar. 14, 2018    
Class A Common Stock | Fiscal 2017 Authorization      
Stock Repurchase Program [Abstract]      
Stock repurchase program, remaining authorized amount   $ 3,000,000,000  
Class A Common Stock | Fiscal 2016 Authorization      
Stock Repurchase Program [Abstract]      
Stock repurchase program, remaining authorized amount   $ 110,000,000  
Class B Common Stock      
Stock Repurchase Program [Abstract]      
Total number of shares repurchased   0  
Dividends [Abstract]      
Cash dividend declared per share   $ 0.18 $ 0.18
Class B Common Stock | Subsequent Event      
Dividends [Abstract]      
Cash dividend declared per share $ 0.18    
Cash dividend payable date Apr. 18, 2018    
Cash dividend record date Mar. 14, 2018