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Equity (Details) - USD ($)
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Class of Stock [Line Items]    
Total cost $ 1,276,200,000 $ 2,108,700,000
Remaining authorized repurchase amount 783,900,000  
Authorized amount 500,000,000.0  
Common Class A and Common Class C    
Class of Stock [Line Items]    
Total cost $ 1,288,300,000  
Class A    
Class of Stock [Line Items]    
Shares repurchased (in shares) 12,588,800  
Average price paid per share (in dollars per share) $ 30.15  
Class C    
Class of Stock [Line Items]    
Shares repurchased (in shares) 29,342,800  
Average price paid per share (in dollars per share) $ 30.97