Equity, Reclassification Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gains (losses) reclassified out of accumulated other comprehensive income (loss)
Net of tax		$ (12)	$ (7)	$ (19)
Net Actuarial Gain (Loss) [Member]
Amortization of items related to defined benefit pension plans:
Reclassification from accumulated other comprehensive income (loss), current period, before tax	[1]	(50)	(48)	(62)
Tax benefit		18	18	22
Gains (losses) reclassified out of accumulated other comprehensive income (loss)
Net of tax		(32)	(30)	(40)
Prior Service (Cost) Credit [Member]
Amortization of items related to defined benefit pension plans:
Reclassification from accumulated other comprehensive income (loss), current period, before tax	[1]	36	36	40
Tax benefit		(13)	(13)	(14)
Gains (losses) reclassified out of accumulated other comprehensive income (loss)
Net of tax		23	23	26
Curtailment and Settlement Loss [Member]
Amortization of items related to defined benefit pension plans:
Reclassification from accumulated other comprehensive income (loss), current period, before tax	[1]	(4)	0	(7)
Tax benefit		1		2
Gains (losses) reclassified out of accumulated other comprehensive income (loss)
Net of tax		(3)		(5)
Net Gain (Loss) on Pension and Other Postretirement Benefits [Member]
Amortization of items related to defined benefit pension plans:
Reclassification from accumulated other comprehensive income (loss), current period, before tax		(18)	(12)	(29)
Tax benefit		$ 6	$ 5	$ 10

[1] These accumulated other comprehensive loss components are included in the computation of net periodic benefit cost, as further discussed in Note 12. Net periodic benefit cost is reflected in operating expenses (excluding depreciation and amortization expense) and general and administrative expenses (excluding depreciation and amortization expense).