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Long-Term Debt - Maturities of Long-term Debt Outstanding (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
2018 $ 2,572  
2019 8,001  
2020 3,047  
2021 2,215  
2022 4,074  
Thereafter 12,350  
Total 32,259 $ 33,323
Parent Company [Member]    
Debt Instrument [Line Items]    
2018 1,499  
2019 1,497  
2021 2,196  
2022 3,790  
Thereafter 6,787  
Total $ 15,769 $ 13,045