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Employee Benefit Plans (Rollforward of Level 3 Plan Assets) (Details) - Pension Plans - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Pension and Other Postretirement Plans    
Fair value of plan assets at beginning of year $ 3,431 $ 3,497
Fair value of plan assets at end of year 3,468 3,431
Level 3    
Pension and Other Postretirement Plans    
Fair value of plan assets at beginning of year 38 35
Actual return on plan assets related to assets held at end of period 1 0
Purchases, sales, other settlements and issuances, net 8 3
Fair value of plan assets at end of year $ 47 $ 38