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Equity Method Investments (Equity Method Investment Balances) (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Equity Method Investments          
Investments $ 2,432 $ 2,623 $ 2,432 $ 2,623  
Income (loss)     $ 20 $ (34) $ (79)
Investment Management          
Equity Method Investments          
Impairment of equity method investment $ 46 $ 53