Shareholders' Equity (Details) - USD ($) shares in Thousands, $ in Millions	6 Months Ended
	Jun. 30, 2017		Jun. 30, 2016
Repurchase Of Shares Of Common Stock [Line Items]
Shares Repurchased	12,462	[1]	28,544	[2]
Dollar Value of Shares Repurchased	$ 826	[1]	$ 2,181	[2]
Remaining Dollar Value of Shares that may be Repurchased	1,089
Value of share repurchases with trade dates prior to current reporting date but with settlement dates subsequent to the current reporting date.	$ 15		$ 115
Number of shares repurchased with trade dates prior to current reporting date but with settlement dates subsequent to the current reporting date.	200		1,400
Value of shares repurchased with trade dates prior to the current reporting quarter, but settlement dates in the current quarter	$ 45
Number of shares repurchased with trade dates prior to the current reporting quarter, but settlement dates in the current quarter	700
December 2015 [Member]
Repurchase Of Shares Of Common Stock [Line Items]
Shares Repurchased	0		13,369
Dollar Value of Shares Repurchased	$ 0		$ 933
Remaining Dollar Value of Shares that may be Repurchased	$ 0
March 2016 [Member]
Repurchase Of Shares Of Common Stock [Line Items]
Shares Repurchased	0		2,823
Dollar Value of Shares Repurchased	$ 0		$ 228
Remaining Dollar Value of Shares that may be Repurchased	$ 0
May 2016 [Member]
Repurchase Of Shares Of Common Stock [Line Items]
Shares Repurchased	0		12,352
Dollar Value of Shares Repurchased	$ 0		$ 1,020
Remaining Dollar Value of Shares that may be Repurchased	$ 0
November 2016 [Member]
Repurchase Of Shares Of Common Stock [Line Items]
Shares Repurchased	12,462		0
Dollar Value of Shares Repurchased	$ 826		$ 0
Remaining Dollar Value of Shares that may be Repurchased	$ 1,089

[1] Includes the effect of $15 million in share repurchases (0.2 million shares) with trade dates on, or prior to, June 30, 2017, but cash settlement dates subsequent to June 30, 2017, and excludes the effect of $45 million in share repurchases (0.7 million shares) with trade dates on, or prior to, December 31, 2016, but cash settlement dates subsequent to December 31, 2016.

[2] (b)Includes the effect of $115 million in share repurchases (1.4 million shares) with trade dates on, or prior to, June 30, 2016, but cash settlement dates subsequent to June 30, 2016.