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Investments - Real estate and other investments included on the Balance Sheet (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Investment [Line Items]    
Limited partnerships and similar investments accounted for using the equity method $ 999 $ 688
Directly owned real estate 164 197
Other investments 1,311 1,034
Other investments    
Investment [Line Items]    
Other investments $ 148 $ 149