Benefit Plans - Obligations and funded status narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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
Decrease in defined benefit net actuarial loss arising during the period	$ (89)
Net actuarial pension benefit losses	$ (2,313)	$ (2,224)
Percentage of underfunded status relating to the primary employee plans	78.00%
Accumulated benefit obligation (ABO)	$ 6,150	6,740
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	5,990	6,420
Company's pension plans with ABO in excess of plan assets, aggregate accumulated benefit obligation	5,930	6,360
Company's pension plans with ABO in excess of plan assets, aggregate fair value of plan assets	5,070	5,890
Defined benefit plans' accrued benefit costs related to certain unfunded and non-qualified plans	$ 135	140
Plan assets and the adjusted fair value of plan assets, recognition period	5 years
Deferred net gain	$ 420
Average remaining service period	10 years
Settlement loss, pretax	$ 203	153	$ 27
Pension benefits | Corporate and Other
Company's Pension Plans with an ABO in excess of plan assets
Settlement loss, pretax	172	122
Postretirement benefits
Components of net periodic cost
Decrease in defined benefit net actuarial loss arising during the period	18
Net actuarial pension benefit losses	200	218
Company's Pension Plans with an ABO in excess of plan assets
Settlement loss, pretax	$ 0	$ 0	$ 0
Defined benefit plan, assumed health care cost trend rates
Weighted average health care cost trend rate	5.90%
Ultimate health care cost trend rate in 2038 and thereafter	4.50%