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Fair Value Measurements Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Recurring measurement | Level 3          
Fair Value, Instruments Classified in Shareholders' Equity Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]          
Fair value of assets measured at fair value on a recurring basis $ 264 $ 368 $ 264 $ 278 $ 211
Recurring measurement | Holding Company | Trading securities          
Fair Value, Instruments Classified in Shareholders' Equity Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]          
Fair value of assets measured at fair value on a recurring basis   365   $ 274  
Nonrecurring measurement          
Fair Value, Instruments Classified in Shareholders' Equity Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]          
Impairment of PP&E   $ 7 2    
Inventory Write-down     6    
Impairment of assets held for sale $ 1   $ 68