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Postretirement Benefit Plans (Textuals) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block        
Other postretirement benefit expense (less than $1 million for the three months ended June 30, 2017) $ 1 $ 1 $ 1 $ 2
Amortization of net actuarial loss (less than $1 million) 1 $ 1 1 1
Employer contribution to defined benefit plan     13 $ 14
Minimum        
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block        
Additional contributions 9   9  
Maximum        
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block        
Additional contributions $ 15   $ 15