Employee Benefit Plans - Fair Value of Plan Assets and Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan, Change in Benefit Obligation
Actuarial loss (gain)	$ (526)	$ 1,100
Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation
Defined benefit plan, benefit obligation period start	20,611	21,856
Service cost	0	0	$ 0
Interest cost	917	884	928
Actuarial loss (gain)	411	(1,061)
Benefits paid, including lump sums and annuities	(1,071)	(1,059)
Participant contributions	0	0
Settlements	(9)	(9)
Defined benefit plan, benefit obligation period end	20,859	20,611	21,856
Defined Benefit Plan, Change in Fair Value of Plan Assets
Defined benefit plan, fair value of plan assets period start	9,374	9,355
Actual gain (loss) on plan assets	687	(132)
Employer contributions	1,320	1,219
Participant contributions	0	0
Benefits paid, including lump sums and annuities	(1,071)	(1,059)
Settlements	(9)	(9)
Defined benefit plan, fair value of plan assets period start end	10,301	9,374	9,355
Defined Benefit Plan, Funded Status of Plan
Defined benefit plan, funded status at end of period	(10,558)	(11,237)
Other Postretirement and Postemployment Benefits
Defined Benefit Plan, Change in Benefit Obligation
Defined benefit plan, benefit obligation period start	3,336	3,487
Service cost	68	62	52
Interest cost	147	141	155
Actuarial loss (gain)	115	(88)
Benefits paid, including lump sums and annuities	(318)	(302)
Participant contributions	31	36
Settlements	0	0
Defined benefit plan, benefit obligation period end	3,379	3,336	3,487
Defined Benefit Plan, Change in Fair Value of Plan Assets
Defined benefit plan, fair value of plan assets period start	884	982
Actual gain (loss) on plan assets	51	(25)
Employer contributions	154	210
Participant contributions	31	36
Benefits paid, including lump sums and annuities	(336)	(319)
Settlements	0	0
Defined benefit plan, fair value of plan assets period start end	784	884	$ 982
Defined Benefit Plan, Funded Status of Plan
Defined benefit plan, funded status at end of period	$ (2,595)	$ (2,452)