Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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 [Abstract]
Shareholders' net income	$ 266	$ 560	$ 813	$ 598	$ 382	$ 456	$ 510	$ 519	$ 2,237	$ 1,867	$ 2,094
Shares:
Weighted average									250,892	255,360	256,149
Common stock equivalents									4,180	4,287	4,443
Total shares									255,072	259,647	260,592
EPS, basic	$ 1.09	$ 2.25	$ 3.2	$ 2.34	$ 1.49	$ 1.79	$ 2	$ 2.04	$ 8.92	$ 7.31	$ 8.17
EPS, effect of dilution									(0.15)	(0.12)	(0.13)
EPS, diluted	$ 1.07	$ 2.21	$ 3.15	$ 2.3	$ 1.47	$ 1.76	$ 1.97	$ 2	$ 8.77	$ 7.19	$ 8.04
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive options									900	2,300	400
Common shares held in Treasury	52,178				39,276				52,178	39,276	39,601