Shareholders' Equity - Narrative (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Jan. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Preferred stock, redemption period			5 years
Repurchase Plan
Class of Stock [Line Items]
Repurchase of common stock			$ 1,600	$ 320
Number of shares repurchased	910,000	3,400,000	35,500,000	8,400,000
Accelerated share repurchase program		$ 200
Common stock, remaining dollar amount authorized for repurchase			$ 640