Unaudited Quarterly Operating Results Unaudited Quarterly Operating Results (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Unaudited Quarterly Operating Results [Abstract]
Operating revenue	$ 1,211	$ 1,327	$ 1,153	$ 1,191	$ 1,178	$ 1,198	$ 1,091	$ 1,119	$ 4,882	$ 4,586	$ 4,432
Operating income	114	378	251	278	238	316	263	267	1,021	1,084	1,036
Net income	$ 44	$ 212	$ 130	$ 151	$ 130	$ 169	$ 129	$ 127	$ 537	$ 555	$ 566