Accumulated Other Comprehensive Income (Loss) - Components of Accumulated Other Comprehensive Income, Net of Tax (Details) - USD ($) $ in Millions		3 Months Ended			6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017		Jun. 30, 2018	Jun. 30, 2017
AOCI Attributable To Parent, Tax [Roll Forward]
Beginning Balance, income taxes		$ (48)	$ 30		$ (39)	$ 26
Changes in value, income taxes		0	8		(6)	18
Amounts reclassified to earnings, income taxes		(4)	(6)		(7)	(12)
Amounts reclassified to retained earnings, income taxes					0
Net change, income taxes		(4)	2		(13)	6
Ending Balance, income taxes		(52)	32		(52)	32
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance, net of tax	[1]				8,734
Total other comprehensive income (loss), net		12	(8)	[2]	45	(15)	[2]
Ending Balance, net of tax		8,639			8,639
Pension and Other Postretirement Liabilities
AOCI Attributable To Parent, Before Tax [Roll Forward]
Beginning Balance, before tax		(1,063)	(867)		(1,102)	(854)
Changes in value, before tax		1	(7)		24	(33)
Amounts reclassified to earnings, before tax		14	14		30	27
Amounts reclassified to retained earnings, before tax					0
Net change, before tax		15	7		54	(6)
Ending Balance, before tax		(1,048)	(860)		(1,048)	(860)
Investments and Other
AOCI Attributable To Parent, Before Tax [Roll Forward]
Beginning Balance, before tax		(3)	1		(6)	(1)
Changes in value, before tax		1	(17)		(3)	(17)
Amounts reclassified to earnings, before tax		0	0		0	2
Amounts reclassified to retained earnings, before tax					7
Net change, before tax		1	(17)		4	(15)
Ending Balance, before tax		(2)	(16)		(2)	(16)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance, net of tax		(1,114)	(836)		(1,147)	(829)
Changes in value, net of tax		2	(16)		15	(32)
Amounts reclassified to earnings, net of tax		10	8		23	17
Amounts reclassified to retained earnings, net of tax					7
Total other comprehensive income (loss), net		12	(8)		45	(15)
Ending Balance, net of tax		$ (1,102)	$ (844)		$ (1,102)	$ (844)

[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.