Quarterly Financial Data (Unaudited) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 2,238	$ 2,287	$ 2,222	$ 2,217	$ 2,223	$ 2,198	$ 2,206	$ 2,211	$ 8,964	$ 8,838	$ 8,753
Operating income	622	572	521	545	496	528	546	542	2,260	2,112	2,078
Income tax expense	169	171	152	167	161	156	162	160	659	639	602
Net income	321	268	$ 238	$ 247	$ 216	$ 245	$ 256	$ 253	$ 1,074	$ 970	$ 964
Incremental increase in other operating revenues - CAF Phase II	$ 31	$ 64