Accumulated Other Comprehensive Income (Loss) - Components of Accumulated Other Comprehensive Income, Net of Tax (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017		Sep. 30, 2018	Sep. 30, 2017
AOCI Attributable To Parent, Tax [Roll Forward]
Beginning Balance, income taxes		$ (52)	$ 32		$ (39)	$ 26
Changes in value, income taxes		0	(6)		(6)	12
Amounts reclassified to earnings, income taxes		(3)	(5)		(10)	(17)
Amounts reclassified to retained earnings, income taxes					0
Net change, income taxes		(3)	(11)		(16)	(5)
Ending Balance, income taxes		(55)	21		(55)	21
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	[1]				8,734
Total other comprehensive income, net		13	20	[2]	58	5	[2]
Ending balance		9,469			9,469
Pension and Other Postretirement Liabilities
AOCI Attributable To Parent, Before Tax [Roll Forward]
Beginning Balance, before tax		(1,048)	(860)		(1,102)	(854)
Changes in value, before tax		0	(9)		24	(42)
Amounts reclassified to earnings, before tax		15	14		45	41
Amounts reclassified to retained earnings, before tax					0
Net change, before tax		15	5		69	(1)
Ending Balance, before tax		(1,033)	(855)		(1,033)	(855)
Investments and Other
AOCI Attributable To Parent, Before Tax [Roll Forward]
Beginning Balance, before tax		(2)	(16)		(6)	(1)
Changes in value, before tax		1	26		(2)	9
Amounts reclassified to earnings, before tax		0	0		0	2
Amounts reclassified to retained earnings, before tax					7
Net change, before tax		1	26		5	11
Ending Balance, before tax		(1)	10		(1)	10
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(1,102)	(844)		(1,147)	(829)
Changes in value, net of tax		1	11		16	(21)
Amounts reclassified to earnings, net of tax		12	9		35	26
Amounts reclassified to retained earnings, net of tax					7
Total other comprehensive income, net		13	20		58	5
Ending balance		$ (1,089)	$ (824)		$ (1,089)	$ (824)

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part I, Item 1 of this report for additional information.

[2] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606) and Accounting Standards Update No. 2017-07, Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. See Note 1 to the financial statements contained in Part I, Item 1 of this report for additional information.