Quarterly Financial Data (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	$ 2,223	$ 2,198	$ 2,206	$ 2,211	$ 2,207	$ 2,188	$ 2,199	$ 2,159	$ 8,838	$ 8,753	$ 8,848
Operating income	496	528	546	542	507	493	525	553	2,112	2,078	1,905
Income tax expense	161	156	162	160	142	139	155	166	639	602	542
Net income	$ 216	$ 245	$ 256	$ 253	$ 236	$ 218	$ 246	$ 264	$ 970	$ 964	$ 849