Employee Benefits - Cash Flows (Details) $ in Millions	Dec. 31, 2018 USD ($)
Retirement Plans
Information about the expected cash flows for retirement and post-retirement plans and impact of the Medicare subsidy
2019, Net payments	$ 91
2020, Net payments	94
2021, Net payments	96
2022, Net payments	99
2023, Net payments	101
2024-2028, Net payments	534
Postretirement Plans
Information about the expected cash flows for retirement and post-retirement plans and impact of the Medicare subsidy
2019, Gross payments	8
2020, Gross payments	7
2021, Gross payments	6
2022, Gross payments	6
2023, Gross payments	5
2024-2028, Gross payments	19
2019, Retiree Contributions	(2)
2020, Retiree Contributions	(2)
2021, Retiree Contributions	(2)
2022, Retiree Contributions	(2)
2023, Retiree Contributions	1
2024-2028, Retiree Contributions	(7)
2019, Medicare subsidy (less than $1 million)	0
2020, Medicare subsidy (less than $1 million)	0
2021, Medicare subsidy (less than $1 million)	0
2022, Medicare subsidy (less than $1 million)	0
2023, Medicare subsidy (less than $1 million)	0
2024-2028, Medicare subsidy (less than $1 million)	0
2019, Net payments	6
2020, Net payments	5
2021, Net payments	4
2022, Net payments	4
2023, Net payments	6
2024-2028, Net payments	$ 12