Shareholders' Equity (Details) - USD ($) shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
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.	$ 5		$ 45
Number of shares repurchased with trade dates during the current reporting date but with settlement dates subsequent to the current reporting date.	100		700
Stock Repurchased During Period, Shares	28,243	[1]	26,561	[2]	67,963	[2]
Stock Repurchased During Period, Value	$ 2,394	[1]	$ 1,915	[2]	$ 5,447	[2]
August 2018 [Domain]
Repurchase Of Shares Of Common Stock [Line Items]
Stock Repurchased During Period, Shares	10,003
November 2017 [Domain]
Repurchase Of Shares Of Common Stock [Line Items]
Stock Repurchased During Period, Shares	18,240		0		0
Stock Repurchased During Period, Value	$ 1,500		$ 0		$ 0
November 2016 [Member]
Repurchase Of Shares Of Common Stock [Line Items]
Stock Repurchased During Period, Shares	0		26,561		1,337
Stock Repurchased During Period, Value	$ 0		$ 1,915		$ 85
December 2015 [Member]
Repurchase Of Shares Of Common Stock [Line Items]
Stock Repurchased During Period, Shares	0		0		13,368
Stock Repurchased During Period, Value	$ 0		$ 0		$ 933
May 2016 [Member]
Repurchase Of Shares Of Common Stock [Line Items]
Stock Repurchased During Period, Shares	0		0		50,435
Stock Repurchased During Period, Value	$ 0		$ 0		$ 4,200
March 2016 [Member]
Repurchase Of Shares Of Common Stock [Line Items]
Stock Repurchased During Period, Shares	0		0		2,823
Stock Repurchased During Period, Value	$ 0		$ 0		$ 229
August 2018 [Member]
Repurchase Of Shares Of Common Stock [Line Items]
Stock Repurchased During Period, Shares			0		0
Stock Repurchased During Period, Value	894		$ 0		$ 0
August 2018 [Member]
Repurchase Of Shares Of Common Stock [Line Items]
Stock Repurchase Program, Authorized Amount	2,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 1,100

[1] 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.

[2] 2017 amount excludes and 2016 amount includes the effect of $45 million in share repurchases (0.7 million shares) with trade dates prior to December 31, 2016 but settlement dates subsequent to December 31, 2016.