Shareholders' Deficit (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2018		Mar. 31, 2017
Repurchase Of Shares Of Common Stock [Line Items]
Shares Repurchased	6,507	[1]	6,849	[2]
Dollar Value of Shares Repurchased	$ 528	[1]	$ 442	[2]
Remaining Dollar Value of Shares that may be Repurchased	972
Value of share repurchases with trade dates prior to current reporting date but with settlement dates subsequent to the current reporting date.	$ 30		$ 26
Number of shares repurchased with trade dates prior to current reporting date but with settlement dates subsequent to the current reporting date.	400		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
November 2016 [Member]
Repurchase Of Shares Of Common Stock [Line Items]
Shares Repurchased	0		6,849
Dollar Value of Shares Repurchased	$ 0		$ 442
Remaining Dollar Value of Shares that may be Repurchased	$ 0
November 2017 [Member]
Repurchase Of Shares Of Common Stock [Line Items]
Shares Repurchased	6,507		0
Dollar Value of Shares Repurchased	$ 528		$ 0
Remaining Dollar Value of Shares that may be Repurchased	$ 972

[1] Includes the effect of $30 million in share repurchases (0.4 million shares) with trade dates prior to March 31, 2018, but cash settlement dates subsequent to March 31, 2018.

[2] Includes the effect of $26 million in share repurchases (0.4 million shares) with trade dates on, or prior to, March 31, 2017, but cash settlement dates subsequent to March 31, 2017 and excludes the effect of $45 million in share purchases (0.7 million shares) with trades dates prior to December 31, 2016, but cash settlement dates subsequent to December 31, 2016.