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Fair Value Measurements - Reconciliation of Net Beginning and Ending Balances Recorded for Net Assets and Liabilities Classified as Level 3 (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance $ 65 $ 68 $ 61 $ 66
Unrealized and realized losses included in net income (9) 20 (2) 29
Settlements of derivative instruments (2) (4) (5) (11)
Ending balance $ 54 $ 84 $ 54 $ 84