Employee Benefit Plans - Benefit Obligations, Fair Value of Plan Assets and Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Change in Benefit Obligation
Benefit obligation at beginning of period	$ 21,696	$ 20,859
Service cost	0	0	$ 0
Interest cost	781	853	917
Actuarial (gain) loss	(1,560)	1,068
Benefits paid, including lump sums and annuities	(1,093)	(1,075)
Participant contributions	0	0
Curtailment	0	0
Settlements	(15)	(9)
Benefit obligation at end of period	19,809	21,696	20,859
Change in Fair Value of Plan Assets
Fair value of plan assets at beginning of period	14,744	10,301
Actual (loss) gain on plan assets	(700)	1,966
Employer contributions	523	3,561
Participant contributions	0	0
Benefits paid, including lump sums and annuities	(1,093)	(1,075)
Settlements	(15)	(9)
Fair value of plan assets at end of period	13,459	14,744	10,301
Funded Status of Plan
Funded status at end of period	(6,350)	(6,952)
Other Postretirement and Postemployment Benefits
Change in Benefit Obligation
Benefit obligation at beginning of period	3,504	3,379
Service cost	85	87	68
Interest cost	126	138	147
Actuarial (gain) loss	(142)	183
Benefits paid, including lump sums and annuities	(306)	(311)
Participant contributions	26	28
Curtailment	(68)	0
Settlements	0	0
Benefit obligation at end of period	3,225	3,504	3,379
Change in Fair Value of Plan Assets
Fair value of plan assets at beginning of period	866	784
Actual (loss) gain on plan assets	(72)	138
Employer contributions	152	254
Participant contributions	26	28
Benefits paid, including lump sums and annuities	(335)	(338)
Settlements	0	0
Fair value of plan assets at end of period	637	866	$ 784
Funded Status of Plan
Funded status at end of period	$ (2,588)	$ (2,638)