Earnings per Share (details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings per Share disclosure
Net income	$ 551	$ 293	$ 595	$ 617	$ 943	$ 716	$ 664	$ 691	$ 2,056	$ 3,014	$ 3,439
Participating share-based awards - allocated income									(15)	(22)	(25)
Net income available to common shareholders -- basic									2,041	2,992	3,414
Net income available to common shareholders -- diluted									$ 2,041	$ 2,992	$ 3,414
Weighted average shares outstanding, basic (in shares)									276.0	288.1	310.6
Weighted average effects of dilutive securities, stock options and performance shares (in shares)									2.6	2.9	3.3
Weighted average shares outstanding, diluted (in shares)									278.6	291.0	313.9
Net income per common share, basic (in dollars per share)	$ 2.00	$ 1.06	$ 2.13	$ 2.19	$ 3.32	$ 2.48	$ 2.27	$ 2.33	$ 7.39	$ 10.39	$ 10.99
Net income per common share, diluted (in dollars per share)	$ 1.98	$ 1.05	$ 2.11	$ 2.17	$ 3.28	$ 2.45	$ 2.24	$ 2.30	$ 7.33	$ 10.28	$ 10.88