Shareholders' Deficit (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017
Repurchase Of Shares Of Common Stock [Line Items]
Shares Repurchased			14,147	[1]	12,462	[2]
Dollar Value of Shares Repurchased			$ 1,171	[1]	$ 826	[2]
Remaining Dollar Value of Shares that may be Repurchased	$ 329		$ 329
Value of share repurchases with trade dates prior to current reporting date but with settlement dates subsequent to the current reporting date.	$ 3	$ 15
Number of shares repurchased with trade dates prior to current reporting date but with settlement dates subsequent to the current reporting date.	30	200
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		12,462
Dollar Value of Shares Repurchased			$ 0		$ 826
Remaining Dollar Value of Shares that may be Repurchased	$ 0		$ 0
November 2017 [Member]
Repurchase Of Shares Of Common Stock [Line Items]
Shares Repurchased			14,147		0
Dollar Value of Shares Repurchased			$ 1,171		$ 0
Remaining Dollar Value of Shares that may be Repurchased	$ 329		$ 329

[1] Includes the effect of $3 million in share repurchases (0.03 million shares) with trade dates prior to June 30, 2018, but cash settlement dates subsequent to June 30, 2018.

[2] 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 purchases (0.7 million shares) with trades dates prior to December 31, 2016, but cash settlement dates subsequent to December 31, 2016.