Unaudited Quarterly Operating Results (Details) (USD $) In Millions, 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
Unaudited Quarterly Operating Results [Abstract]
Operating revenue	$ 458	$ 867	$ 595	$ 417	$ 397	$ 786	$ 537	$ 372	$ 2,337	$ 2,092	$ 2,145
Operating income	34	307	145	55	(69)	305	141	58	541	435	602
Net income	$ (9)	$ 168	$ 62	$ 6	$ (83)	$ 164	$ 59	$ 5	$ 227	$ 145	$ 258