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Fair Value Measurements (Narrative) (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Assets transferred between Level 1 and Level 2 $ 0  
Adjustments to acquisition related contingent consideration liability 3,000,000  
Increase in fair value of contingent consideration due to 5% increase in projections 18,000,000  
Decrease in fair value of contingent consideration due to 5% decrease in projections 19,000,000  
Equity method investments $ 405,000,000 $ 389,000,000
Minimum    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Revenue target period (in years) 2 years  
Maximum    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Revenue target period (in years) 4 years  
Private Equity Funds    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity method investments $ 76,000,000 $ 79,000,000
Money Market Funds    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Share price (Rand) $ 1.00