Employee Benefit Plans - Net Periodic Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 0	$ 0	$ 0
Interest cost	853	917	884
Expected return on plan assets	(1,143)	(902)	(879)
Amortization of prior service credit	0	0	0
Recognized net actuarial loss	262	233	232
Settlements	3	3	3
Net periodic cost	(25)	251	240
Other Postretirement and Postemployment Benefits
Defined Benefit Plan Disclosure
Service cost	87	68	62
Interest cost	138	147	141
Expected return on plan assets	(69)	(74)	(81)
Amortization of prior service credit	(26)	(26)	(26)
Recognized net actuarial loss	32	24	24
Settlements	0	0	0
Net periodic cost	$ 162	$ 139	$ 120