Quarterly Financial Data (Unaudited) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating quarterly financial data
Operating revenues	$ 2,208	$ 2,226	$ 2,223	$ 2,253	$ 2,238	$ 2,287	$ 2,222	$ 2,217	$ 8,910	$ 8,964	$ 8,838
Operating income	519	576	602	625	622	572	521	545	2,322	2,260	2,112
Income tax expense	152	159	179	188	169	171	152	167	678	659	639
Net income	238	$ 255	$ 288	$ 304	321	268	$ 238	$ 247	$ 1,085	$ 1,074	$ 970
Severance Costs	$ 78
CAF Phase 2 Support
Operating quarterly financial data
Incremental increase in other operating revenues - CAF Phase II					$ 31	$ 64