Quarterly Financial Information (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Summarized quarterly financial information
Operating revenues	$ 3,696	$ 5,428	$ 4,521	$ 4,012	$ 3,771	$ 5,320	$ 4,208	$ 4,157	$ 17,657	$ 17,456	$ 15,743
Operating Income	589	1,652	1,136	854	470	1,459	951	922	4,231	3,802	3,268
Consolidated Net Income After Dividends on Preferred and Preference Stock of Subsidiaries	$ 261	$ 916	$ 604	$ 422	$ 153	$ 817	$ 510	$ 495	$ 2,203	$ 1,975	$ 1,643
Basic Earnings	$ 0.30	$ 1.07	$ 0.71	$ 0.50	$ 0.18	$ 0.98	$ 0.62	$ 0.60	$ 2.57	$ 2.37	$ 2.07
Dividends	$ 0.4725	$ 0.4725	$ 0.4725	$ 0.4550	$ 0.4550	$ 0.4550	$ 0.4550	$ 0.4375	$ 1.8725	$ 1.8025	$ 1.7325
Trading price range per common share high	$ 46.69	$ 43.09	$ 40.87	$ 38.79	$ 38.62	$ 37.73	$ 35.45	$ 33.73
Trading price range per common share low	$ 41.00	$ 35.73	$ 37.43	$ 36.51	$ 37.10	$ 33.00	$ 32.04	$ 30.85