Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated other comprehensive loss
Balance at beginning of period			$ 13,399
Accumulated other comprehensive income (loss) by component
Other comprehensive income (loss) before reclassifications	$ 18	$ (4)	20	$ (9)
Amounts reclassified from accumulated other comprehensive income	38	30	103	88
Other comprehensive income	56	26	123	79
Balance at end of period	12,960	13,892	12,960	13,892
Defined benefit plan
Accumulated other comprehensive income (loss) by component
Amounts reclassified from accumulated other comprehensive income	38	30	103	88
Defined benefit plan | Pension plans
Accumulated other comprehensive loss
Balance at beginning of period	(1,836)	(1,663)	(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	34	26	93	78
Other comprehensive income	34	26	93	78
Balance at end of period	(1,802)	(1,637)	(1,802)	(1,637)
Defined benefit plan | Post-retirement benefit plans
Accumulated other comprehensive loss
Balance at beginning of period	(156)	(174)	(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	4	4	10	10
Other comprehensive income	4	4	10	10
Balance at end of period	(152)	(170)	(152)	(170)
Foreign currency translation adjustment and other
Accumulated other comprehensive loss
Balance at beginning of period	(58)	(44)	(60)	(39)
Accumulated other comprehensive income (loss) by component
Other comprehensive income (loss) before reclassifications	18	(4)	20	(9)
Amounts reclassified from accumulated other comprehensive income	0	0	0	0
Other comprehensive income	18	(4)	20	(9)
Balance at end of period	(40)	(48)	(40)	(48)
Accumulated other comprehensive income
Accumulated other comprehensive loss
Balance at beginning of period	(2,050)	(1,881)	(2,117)	(1,934)
Accumulated other comprehensive income (loss) by component
Other comprehensive income			123	79
Balance at end of period	$ (1,994)	$ (1,855)	$ (1,994)	$ (1,855)