Quarterly Results (unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues	$ 8,759	$ 8,936	$ 8,860	$ 8,684	$ 8,792	$ 8,465	$ 8,787	$ 8,463	$ 35,239	$ 34,507	$ 33,315
Net income available to common shareholders	$ 795	$ 750	$ 614	$ 587	$ 810	$ 310	$ 434	$ 709	$ 2,746	$ 2,263	$ 2,306
Net income available to common shareholders per common share - Basic (in dollars per share)	$ 1.89	$ 1.77	$ 1.41	$ 1.31	$ 1.79	$ 0.67	$ 0.93	$ 1.49	$ 6.37	$ 4.87	$ 4.71
Net income available to common shareholders per common share - Diluted (in dollars per share)	$ 1.86	$ 1.74	$ 1.39	$ 1.30	$ 1.76	$ 0.66	$ 0.92	$ 1.47	$ 6.27	$ 4.81	$ 4.68