Benefit Plans - Obligations and funded status narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Retirement Benefits [Abstract]
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)	$ 7,020	$ 6,150
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	6,730	5,990
Company's pension plans with ABO in excess of plan assets, aggregate accumulated benefit obligation	6,620	5,930
Company's pension plans with ABO in excess of plan assets, aggregate fair value of plan assets	5,790	5,070
Defined benefit plans' accrued benefit costs related to certain unfunded and non-qualified plans	$ 137	$ 135
Postretirement benefits
Defined benefit plan, assumed health care cost trend rates
Weighted average health care cost trend rate	7.00%
Ultimate health care cost trend rate in 2038 and thereafter	4.50%