Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accumulated other comprehensive loss
Balance at beginning of period			$ 23,491
Other comprehensive income (loss) before reclassifications			(161)	$ 20
Amounts reclassified from accumulated other comprehensive income			109	103
Other comprehensive income (loss)	$ 36	$ 56	(52)	123
Cumulative effect of adoption of ASU					$ (407)
Balance at end of period	22,803	12,960	22,803	12,960
Defined benefit plan | Pension plans
Accumulated other comprehensive loss
Balance at beginning of period			(1,731)	(1,895)
Other comprehensive income (loss) before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive income			97	93
Other comprehensive income (loss)			97	93
Cumulative effect of adoption of ASU					(375)
Balance at end of period	(2,009)	(1,802)	(2,009)	(1,802)
Defined benefit plan | Post-retirement benefit plans
Accumulated other comprehensive loss
Balance at beginning of period			(235)	(162)
Other comprehensive income (loss) before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive income			12	10
Other comprehensive income (loss)			12	10
Cumulative effect of adoption of ASU					(32)
Balance at end of period	(255)	(152)	(255)	(152)
Foreign currency translation adjustment and other
Accumulated other comprehensive loss
Balance at beginning of period			(29)	(60)
Other comprehensive income (loss) before reclassifications			(161)	20
Amounts reclassified from accumulated other comprehensive income			0	0
Other comprehensive income (loss)			(161)	20
Cumulative effect of adoption of ASU					$ 0
Balance at end of period	(190)	(40)	(190)	(40)
Accumulated other comprehensive income
Accumulated other comprehensive loss
Balance at beginning of period	(2,490)	(2,050)	(1,995)	(2,117)
Other comprehensive income (loss)	36	56	(52)	123
Balance at end of period	$ (2,454)	$ (1,994)	$ (2,454)	$ (1,994)