Shareholders' Equity (Details) - USD ($) shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Repurchase Of Shares Of Common Stock [Line Items]
Value of share repurchases with trade dates during the current reporting date but with settlement dates subsequent to the current reporting date.	$ 11				$ 5
Number of shares repurchased with trade dates during the current reporting date but with settlement dates subsequent to the current reporting date.	100				100
Stock Repurchased During Period, Shares	2,419	[1]	7,788	[1]	28,243
Stock Repurchased During Period, Value	$ 250	[1]	$ 810	[1]	$ 2,394
Defined Benefit Plan, Amortization of Gain (Loss)	14		2
Amortization of prior service cost	4		5
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement	$ 4		$ 3
November 2019 [Member]
Repurchase Of Shares Of Common Stock [Line Items]
Stock Repurchased During Period, Shares	2,419		0		0
Stock Repurchased During Period, Value	$ 250		$ 0		$ 0
November 2017 [Member]
Repurchase Of Shares Of Common Stock [Line Items]
Stock Repurchased During Period, Shares	0		0		18,240
Stock Repurchased During Period, Value	$ 0		$ 0		$ 1,500
August 2018 [Member]
Repurchase Of Shares Of Common Stock [Line Items]
Stock Repurchased During Period, Shares	0		7,788		10,003
Stock Repurchased During Period, Value	$ 0		$ 810		$ 894
November 2019 [Member]
Repurchase Of Shares Of Common Stock [Line Items]
Stock Repurchase Program, Authorized Amount	2,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 1,750

[1] 2019 amount excludes and 2018 amount includes the effect of $5 million in share repurchases (0.1 million shares) with trade dates on, or prior to, December 31, 2018, but settlement dates subsequent to December 31, 2018.