Benefit Plans - Obligations and funded status narrative (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019 pension_plan	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Retirement Benefits [Abstract]
Number of pension plans | pension_plan	2
Maximum annual medical cost inflation after retirement for Company's share of retiree medical benefits cost for non Medicare-eligible retirees (as a percent)		5.00%
Pension benefits
Components of net periodic cost
Accumulated benefit obligation (ABO)		$ 7,550	$ 7,020
Company's Pension Plans with an ABO in excess of plan assets
Company's pension plans with ABO in excess of plan assets, aggregate projected benefit obligation		7,330	6,730
Company's pension plans with ABO in excess of plan assets, aggregate accumulated benefit obligation		7,130	6,620
Company's pension plans with ABO in excess of plan assets, aggregate fair value of plan assets		6,560	5,790
Defined benefit plans' accrued benefit costs related to certain unfunded and non-qualified plans		$ 139	$ 137
Postretirement benefits
Defined benefit plan, assumed health care cost trend rates
Weighted average health care cost trend rate		6.80%
Ultimate health care cost trend rate in 2038 and thereafter		4.50%