Employee Benefit Plans - Benefit Obligations, Fair Value of Plan Assets and Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation
Benefit obligation, beginning of period	$ 20,859	$ 20,611
Service cost	0	0	$ 0
Interest cost	853	917	884
Actuarial loss (gain)	1,068	411
Benefits paid, including lump sums and annuities	(1,075)	(1,071)
Participant contributions	0	0
Settlements	(9)	(9)
Benefit obligation, end of period	21,696	20,859	20,611
Defined Benefit Plan, Change in Fair Value of Plan Assets
Fair value of plan assets, beginning of period	10,301	9,374
Actual gain (loss) on plan assets	1,966	687
Employer contributions	3,561	1,320
Participant contributions	0	0
Benefits paid, including lump sums and annuities	(1,075)	(1,071)
Settlements	(9)	(9)
Fair value of plan assets, end of period	14,744	10,301	9,374
Defined Benefit Plan, Funded Status of Plan
Funded status at end of period	(6,952)	(10,558)
Other Postretirement and Postemployment Benefits
Defined Benefit Plan, Change in Benefit Obligation
Benefit obligation, beginning of period	3,379	3,336
Service cost	87	68	62
Interest cost	138	147	141
Actuarial loss (gain)	183	115
Benefits paid, including lump sums and annuities	(311)	(318)
Participant contributions	28	31
Settlements	0	0
Benefit obligation, end of period	3,504	3,379	3,336
Defined Benefit Plan, Change in Fair Value of Plan Assets
Fair value of plan assets, beginning of period	784	884
Actual gain (loss) on plan assets	138	51
Employer contributions	254	154
Participant contributions	28	31
Benefits paid, including lump sums and annuities	(338)	(336)
Settlements	0	0
Fair value of plan assets, end of period	866	784	$ 884
Defined Benefit Plan, Funded Status of Plan
Funded status at end of period	$ (2,638)	$ (2,595)