Quarterly Results (unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 8,691	$ 9,028	$ 8,982	$ 8,952	$ 8,759	$ 8,936	$ 8,860	$ 8,684	$ 35,653	$ 35,239	$ 34,507
Net income applicable to common shareholders	$ 460	$ 621	$ 326	$ 648	$ 795	$ 750	$ 614	$ 587	$ 2,055	$ 2,746	$ 2,263
Net income applicable to common shareholders earnings per common share - Basic (in dollars per share)	$ 1.19	$ 1.56	$ 0.80	$ 1.56	$ 1.89	$ 1.77	$ 1.41	$ 1.31	$ 5.12	$ 6.37	$ 4.87
Net income applicable to common shareholders earnings per common share - Diluted (in dollars per share)	$ 1.18	$ 1.54	$ 0.79	$ 1.53	$ 1.86	$ 1.74	$ 1.39	$ 1.30	$ 5.05	$ 6.27	$ 4.81