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Fair Value Measurements Changes in Fair Value Level 3 (Details) - Recurring measurement - Level 3 - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, [Roll Forward]        
Fair value of assets measured at fair value on a recurring basis $ 265 $ 222 $ 212 $ 283
Net realized and unrealized gains(1) [1] 51 10    
Purchases 5 50    
Transfers out (6) (127)    
Transfers in 3 6    
Unrealized gain (loss) included in earnings related to Level 3 assets still held at period end $ 51 $ (6)    
[1] Includes net unrealized gains (losses) of $51 million and $(6) million for the nine months ended September 30, 2017 and 2016, respectively, relating to investments still held at September 30 of each respective period and which are included in net gain (loss) from investment activities in the condensed consolidated statements of operations.