Background and Basis of Presentation (Share Repurchase Table) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended	3 Months Ended		12 Months Ended	33 Months Ended
	Jan. 31, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Mar. 31, 2018
Equity, Class of Treasury Stock [Line Items]
Repurchase of common stock (shares)		8.0
Aggregate cost of shares repurchased		$ 513		$ 2,917
Average price of repurchased shares, per share (usd per share)		$ 64.33
July 2015 Share Repurchase Program [Member]
Equity, Class of Treasury Stock [Line Items]
Repurchase of common stock (shares)	0.3		7.7		58.7
Aggregate cost of shares repurchased	$ 18		$ 551
Average price of repurchased shares, per share (usd per share)	$ 71.68		$ 71.77		$ 68.15