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Long-Term Debt - Scheduled Principal Payments on Long-Term Debt (Details)
$ in Millions
Dec. 31, 2017
USD ($)
Debt Instrument [Line Items]  
2018 $ 3,071
2019 2,048
2020 2,706
2021 2,624
2022 1,426
All subsequent years 2,458
Key [Member]  
Debt Instrument [Line Items]  
2018 746
2019 0
2020 1,310
2021 1,381
2022 0
All subsequent years 483
Subsidiaries [Member]  
Debt Instrument [Line Items]  
2018 2,325
2019 2,048
2020 1,396
2021 1,243
2022 1,426
All subsequent years $ 1,975