Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated other comprehensive loss
Balance at beginning of period			$ 13,399
Accumulated other comprehensive income (loss) by component
Other comprehensive income	$ 36	$ 26	67	$ 53
Balance at end of period	13,086	13,985	13,086	13,985
Defined benefit plan
Accumulated other comprehensive income (loss) by component
Amounts reclassified from accumulated other comprehensive income	32	30	65	58
Defined benefit plan | Pension plans
Accumulated other comprehensive loss
Balance at beginning of period	(1,865)	(1,690)	(1,895)	(1,715)
Accumulated other comprehensive income (loss) by component
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income	29	27	59	52
Other comprehensive income	29	27	59	52
Balance at end of period	(1,836)	(1,663)	(1,836)	(1,663)
Defined benefit plan | Post-retirement benefit plans
Accumulated other comprehensive loss
Balance at beginning of period	(159)	(177)	(162)	(180)
Accumulated other comprehensive income (loss) by component
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income	3	3	6	6
Other comprehensive income	3	3	6	6
Balance at end of period	(156)	(174)	(156)	(174)
Foreign currency translation adjustment and other
Accumulated other comprehensive loss
Balance at beginning of period	(62)	(40)	(60)	(39)
Accumulated other comprehensive income (loss) by component
Other comprehensive income (loss) before reclassifications	4	(4)	2	(5)
Amounts reclassified from accumulated other comprehensive income	0	0	0	0
Other comprehensive income	4	(4)	2	(5)
Balance at end of period	(58)	(44)	(58)	(44)
Accumulated other comprehensive income
Accumulated other comprehensive loss
Balance at beginning of period	(2,086)	(1,907)	(2,117)	(1,934)
Accumulated other comprehensive income (loss) by component
Other comprehensive income (loss) before reclassifications	4	(4)	2	(5)
Amounts reclassified from accumulated other comprehensive income	32	30	65	58
Other comprehensive income	36	26	67	53
Balance at end of period	$ (2,050)	$ (1,881)	$ (2,050)	$ (1,881)