Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Accumulated other comprehensive loss
Balance at beginning of period	$ 23,491
Accumulated other comprehensive income (loss) by component
Other comprehensive income (loss) before reclassifications	79	$ (2)
Amounts reclassified from accumulated other comprehensive income	35	33
Other comprehensive income	114	31
Cumulative effect of adoption of ASU			$ (407)
Balance at end of period	23,443	13,306
Defined benefit plan | Pension plans
Accumulated other comprehensive loss
Balance at beginning of period	(1,731)	(1,895)
Accumulated other comprehensive income (loss) by component
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income	31	30
Other comprehensive income	31	30
Cumulative effect of adoption of ASU			(375)
Balance at end of period	(2,075)	(1,865)
Defined benefit plan | Post-retirement benefit plans
Accumulated other comprehensive loss
Balance at beginning of period	(235)	(162)
Accumulated other comprehensive income (loss) by component
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income	4	3
Other comprehensive income	4	3
Cumulative effect of adoption of ASU			(32)
Balance at end of period	(263)	(159)
Foreign currency translation adjustment and other
Accumulated other comprehensive loss
Balance at beginning of period	(29)	(60)
Accumulated other comprehensive income (loss) by component
Other comprehensive income (loss) before reclassifications	79	(2)
Amounts reclassified from accumulated other comprehensive income	0	0
Other comprehensive income	79	(2)
Cumulative effect of adoption of ASU			$ 0
Balance at end of period	50	(62)
Accumulated other comprehensive income
Accumulated other comprehensive loss
Balance at beginning of period	(1,995)	(2,117)
Accumulated other comprehensive income (loss) by component
Other comprehensive income	114	31
Balance at end of period	$ (2,288)	$ (2,086)