Supplementary Quarterly Financial Information (Unaudited) (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
Quarterly Financial Information Disclosure [Abstract]
Operating Revenues	$ 1,234	$ 1,357	$ 1,281	$ 1,410	$ 1,256	$ 1,457	$ 1,265	$ 1,219	$ 5,282	$ 5,197	$ 5,291
Operating Income (Loss)	250	272	259	271	225	338	202	235	1,052	1,000	1,028
Net Income	$ 129	$ 136	$ 130	$ 137	$ 101	$ 180	$ 90	$ 116	$ 532	$ 487	$ 486