Quarterly Financial Data (Unaudited) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								9 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, 2011	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 2,207	$ 2,188	$ 2,199	$ 2,159	$ 2,210	$ 2,183	$ 2,195	$ 2,260	$ 6,635	$ 8,753	$ 8,848
Operating income	507	493	525	553	525	459	455	466	1,199	2,078	1,905
Income tax expense	142	139	155	166	160	133	113	136	349	602	542
Net Income (Loss) Attributable to Parent	$ 236	$ 218	$ 246	$ 264	$ 241	$ 212	$ 178	$ 218	$ 543	$ 964	$ 849