Earnings per Share (details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Earnings per Share disclosure
Net income	$ 691	$ 833
Participating share-based awards - allocated income	(5)	(6)
Net income available to common shareholders -- basic	686	827
Net income available to common shareholders -- diluted	$ 686	$ 827
Weighted average shares outstanding, basic	294.2	320.8
Weighted average effects of dilutive securities, stock options and performance shares (in shares)	3.7	3.7
Weighted average shares outstanding, diluted	297.9	324.5
Net income per common share, basic	$ 2.33	$ 2.58
Net income per common share, diluted	$ 2.30	$ 2.55