Pension and Other Postretirement Employee Benefit Plans - Estimated Future Benefit Payments (Details) $ in Millions	Dec. 31, 2018 USD ($)
Defined benefit pension plans
Defined benefit pension and OPEB before Medicare
2019	$ 939
2020	932
2021	921
2022	920
2023	919
Years 2024–2028	4,529
OPEB plans
Defined benefit pension and OPEB before Medicare
2019	62
2020	60
2021	57
2022	55
2023	52
Years 2024–2028	223
Medicare Part D subsidy
2019	1
2020	1
2021	1
2022	1
2023	0
Years 2024–2028	$ 2

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