Shareholder's Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions					12 Months Ended
	Dec. 26, 2017	Mar. 09, 2017	Nov. 03, 2016	May 07, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 01, 2018	Oct. 26, 2017
Accelerated Share Repurchases [Line Items]
Value of shares repurchased and placed in treasury		$ 150	$ 200		$ 923	$ 687	$ 1,491
Number of shares repurchased and placed in treasury	7,821,666	3,986,647	5,216,025		24,400,000	(17,000,000)	34,300,000
Stock Repurchase Program, Authorized Amount	$ 500								$ 800
Number of warrants issued and outstanding				26,050,846
Percentage of issued warrants to total shares issued and outstanding				9.99%
Exercise price of warrants (usd per share)				$ 48.75
Dividend per share, cash paid (usd per share)				$ 0.01
Fair value of warrants issued				$ 94
Subsequent event
Accelerated Share Repurchases [Line Items]
Stock Repurchase Program, Authorized Amount								$ 500