Employee Benefits - Cash Flows (Details) $ in Millions	Dec. 31, 2017 USD ($)
Retirement Plans
Information about the expected cash flows for retirement and post-retirement plans and impact of the Medicare subsidy
2018, Net payments	$ 88
2019, Net payments	90
2020, Net payments	93
2021, Net payments	96
2022, Net payments	99
2023-2027, Net payments	527
Postretirement Plans
Information about the expected cash flows for retirement and post-retirement plans and impact of the Medicare subsidy
2018, Gross payments	9
2019, Gross payments	8
2020, Gross payments	8
2021, Gross payments	7
2022, Gross payments	6
2023-2027, Gross payments	24
2018, Retiree Contributions	(3)
2019, Retiree Contributions	(3)
2020, Retiree Contributions	(2)
2021, Retiree Contributions	(2)
2022, Retiree Contributions	(2)
2023-2027, Retiree Contributions	(9)
2018, Medicare subsidy	0
2019, Medicare subsidy	0
2020, Medicare subsidy	0
2021, Medicare subsidy	0
2022, Medicare subsidy	0
2023-2027, Medicare subsidy	0
2018, Net payments	6
2019, Net payments	5
2020, Net payments	6
2021, Net payments	5
2022, Net payments	4
2023-2027, Net payments	$ 15