Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 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			(160)	$ 2
Amounts reclassified from accumulated other comprehensive income			72	65
Other comprehensive (loss) income	$ (202)	$ 36	(88)	67
Cumulative effect of adoption of ASU					$ (407)
Balance at end of period	22,989	13,086	22,989	13,086
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			65	59
Other comprehensive (loss) income			65	59
Cumulative effect of adoption of ASU					(375)
Balance at end of period	(2,041)	(1,836)	(2,041)	(1,836)
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			7	6
Other comprehensive (loss) income			7	6
Cumulative effect of adoption of ASU					(32)
Balance at end of period	(260)	(156)	(260)	(156)
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			(160)	2
Amounts reclassified from accumulated other comprehensive income			0	0
Other comprehensive (loss) income			(160)	2
Cumulative effect of adoption of ASU					$ 0
Balance at end of period	(189)	(58)	(189)	(58)
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 (loss) income			(88)	67
Balance at end of period	$ (2,490)	$ (2,050)	$ (2,490)	$ (2,050)