Employee Benefit Plans - Balance Sheet Position (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Pension and Other Postretirement Defined Benefit Plans, Liabilities
Noncurrent liabilities	$ (9,810)	$ (13,378)
Pension Benefits
Pension and Other Postretirement Defined Benefit Plans, Liabilities
Current liabilities	(32)	(30)
Noncurrent liabilities	(6,920)	(10,528)
Total liabilities	(6,952)	(10,558)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, before Tax
Net actuarial loss	(8,495)	(8,515)
Prior service credit	0	0
Total accumulated other comprehensive loss, pre-tax	(8,495)	(8,515)
Other Postretirement and Postemployment Benefits
Pension and Other Postretirement Defined Benefit Plans, Liabilities
Current liabilities	(121)	(125)
Noncurrent liabilities	(2,517)	(2,470)
Total liabilities	(2,638)	(2,595)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, before Tax
Net actuarial loss	(651)	(570)
Prior service credit	56	82
Total accumulated other comprehensive loss, pre-tax	$ (595)	$ (488)