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Pension and Other Postretirement Benefits (Additional Information About Pension Plan Assets Valued Using Net Asset Value) (Details) - Fair Value Measured at Net Asset Value Per Share - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Redemption Notice Period   1 day  
Investment funds – fixed income      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Defined benefit plan assets at fair value   $ 193 $ 210
Redemption Notice Period 1 day 1 day  
Investment funds – global equity      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Defined benefit plan assets at fair value   $ 96 57
Investment funds – emerging markets      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Defined benefit plan assets at fair value   $ 23 20
Redemption Notice Period 30 days 30 days  
Hedge funds      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Defined benefit plan assets at fair value   $ 328 $ 282
Minimum | Investment funds – global equity      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Redemption Notice Period 1 day 1 day  
Minimum | Hedge funds      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Redemption Notice Period 1 day 3 days  
Maximum | Investment funds – fixed income      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Redemption Notice Period 5 days    
Maximum | Investment funds – global equity      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Redemption Notice Period 15 days 30 days  
Maximum | Hedge funds      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Redemption Notice Period 30 days 45 days