Unaudited Quarterly Operating Results Unaudited Quarterly Operating Results (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Unaudited Quarterly Operating Results [Abstract]
Operating revenue	$ 1,302	$ 1,398	$ 1,215	$ 1,232	$ 1,211	$ 1,327	$ 1,153	$ 1,191	$ 5,147	$ 4,882	$ 4,586
Operating income	267	389	311	297	114	378	251	278	1,264	1,021	1,084
Net income	$ 140	$ 216	$ 166	$ 160	$ 44	$ 212	$ 130	$ 151	$ 682	$ 537	$ 555