Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Accumulated other comprehensive income (loss) by component
Balance at the beginning of the period	$ (1,881)	$ (1,959)	$ (1,934)	$ (2,017)
Other comprehensive income (loss) before reclassifications	(4)	(10)	(9)	(10)
Amounts reclassified from accumulated other comprehensive income	30	28	88	86
Other comprehensive income	26	18	79	76
Balance at the end of the period	(1,855)	(1,941)	(1,855)	(1,941)
Defined benefit plan | Pension plans
Accumulated other comprehensive income (loss) by component
Balance at the beginning of the period	(1,663)	(1,668)	(1,715)	(1,720)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income	26	24	78	76
Other comprehensive income	26	24	78	76
Balance at the end of the period	(1,637)	(1,644)	(1,637)	(1,644)
Defined benefit plan | Post-retirement benefit plans
Accumulated other comprehensive income (loss) by component
Balance at the beginning of the period	(174)	(266)	(180)	(272)
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 the end of the period	(170)	(262)	(170)	(262)
Foreign currency translation adjustment and other
Accumulated other comprehensive income (loss) by component
Balance at the beginning of the period	(44)	(25)	(39)	(25)
Other comprehensive income (loss) before reclassifications	(4)	(10)	(9)	(10)
Amounts reclassified from accumulated other comprehensive income	0	0	0	0
Other comprehensive income	(4)	(10)	(9)	(10)
Balance at the end of the period	$ (48)	$ (35)	$ (48)	$ (35)