Shareholders' Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions					6 Months Ended		12 Months Ended
	Aug. 07, 2018	Mar. 26, 2018	Dec. 26, 2017	May 07, 2013	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Feb. 01, 2018
Class of Stock [Line Items]
Share repurchase authorization, amount								$ 500
Payment for share repurchases			$ 500		$ 500	$ 423
Shares delivered (in shares)		1,947,413	7,821,666		(11,600,000)		(24,400,000)
Number of warrants issued and outstanding (in shares)				26,050,846
Warrants issued allowing purchase of shares, percentage of common stock issued and outstanding				9.99%
Exercise price of warrants (usd per share)				$ 48.75
Cash dividends, per share (usd per share)				$ 0.01
Warrants net share settled, fair value				$ 94
Number of warrants exercised					0
Subsequent Event
Class of Stock [Line Items]
Payment for share repurchases	$ 88
Shares delivered (in shares)	1,808,900