Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated other comprehensive loss
Balance at beginning of period	$ 13,399
Accumulated other comprehensive income (loss) by component
Other comprehensive income	31	$ 27
Balance at end of period	13,306	14,036
Defined benefit plan
Accumulated other comprehensive income (loss) by component
Amounts reclassified from accumulated other comprehensive income	33	28
Defined benefit plan | Pension plans
Accumulated other comprehensive loss
Balance at beginning of period	(1,895)	(1,715)
Accumulated other comprehensive income (loss) by component
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income	30	25
Other comprehensive income	30	25
Balance at end of period	(1,865)	(1,690)
Defined benefit plan | Post-retirement benefit plans
Accumulated other comprehensive loss
Balance at beginning of period	(162)	(180)
Accumulated other comprehensive income (loss) by component
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income	3	3
Other comprehensive income	3	3
Balance at end of period	(159)	(177)
Foreign currency translation adjustment and other
Accumulated other comprehensive loss
Balance at beginning of period	(60)	(39)
Accumulated other comprehensive income (loss) by component
Other comprehensive income (loss) before reclassifications	(2)	(1)
Amounts reclassified from accumulated other comprehensive income	0	0
Other comprehensive income	(2)	(1)
Balance at end of period	(62)	(40)
Accumulated other comprehensive income
Accumulated other comprehensive loss
Balance at beginning of period	(2,117)	(1,934)
Accumulated other comprehensive income (loss) by component
Other comprehensive income (loss) before reclassifications	(2)	(1)
Amounts reclassified from accumulated other comprehensive income	33	28
Other comprehensive income	31	27
Balance at end of period	$ (2,086)	$ (1,907)