Quarterly Results (unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 9,278	$ 9,221	$ 9,164	$ 8,871	$ 8,691	$ 9,028	$ 8,982	$ 8,952	$ 36,534	$ 35,653	$ 35,239
Net income applicable to common shareholders	$ 811	$ 491	$ 242	$ 217	$ 460	$ 621	$ 326	$ 648	$ 1,761	$ 2,055	$ 2,746
Net income applicable to common shareholders earnings per common share - Basic (in dollars per share)	$ 2.20	$ 1.32	$ 0.65	$ 0.57	$ 1.19	$ 1.56	$ 0.80	$ 1.56	$ 4.72	$ 5.12	$ 6.37
Net income applicable to common shareholders earnings per common share - Diluted (in dollars per share)	$ 2.18	$ 1.31	$ 0.64	$ 0.57	$ 1.18	$ 1.54	$ 0.79	$ 1.53	$ 4.67	$ 5.05	$ 6.27