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Income Taxes - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended 12 Months Ended 24 Months Ended
Apr. 01, 2017
Sep. 30, 2018
Mar. 31, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Mar. 31, 2017
Income Taxes [Line Items]                
Effect of adjustments to the provisional amount as a result of Tax Act   $ (4)   $ 0 $ (4) $ 0    
Foreign tax credits acquired   36     36      
Tax Cuts and Jobs Act of 2017, incomplete accounting, transition tax for accumulated foreign earnings, tax credit realized         $ 23      
Realization assessment period         3 years      
Accrued interest and penalties, uncertain tax positions   11 $ 7 $ 37        
Interest and penalties related to uncertain tax positions         $ 1   $ 1  
Accrued interest, uncertain tax positions   1     1   3  
Unrecognized tax benefits that would impact effective tax rate   15     $ 15   31  
United Kingdom [Member]                
Income Taxes [Line Items]                
Corporate tax rate 19.00%       19.00%     20.00%
United States [Member]                
Income Taxes [Line Items]                
Effect of adjustments to the provisional amount as a result of Tax Act             $ 61  
Radian [Member]                
Income Taxes [Line Items]                
Foreign tax credits acquired   $ 13     $ 13