Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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	$ 4,476	$ 4,554	$ 4,419	$ 4,451	$ 4,438	$ 4,514	$ 4,541	$ 4,538	$ 17,900	$ 18,031	$ 18,095
OPERATING INCOME	751	656	549	649	483	619	655	653	2,605	2,410	1,453
Net income (loss)	$ 338	$ 205	$ 143	$ 192	$ 188	$ 188	$ 193	$ 203	$ 878	$ 772	$ (239)
Basic earnings (loss) per common share (in dollars per share)	$ 0.62	$ 0.37	$ 0.26	$ 0.34	$ 0.33	$ 0.33	$ 0.34	$ 0.35	$ 1.58	$ 1.36	$ (0.40)
Diluted earnings (loss) per common share (in dollars per share)	$ 0.62	$ 0.37	$ 0.26	$ 0.34	$ 0.33	$ 0.33	$ 0.34	$ 0.35	$ 1.58	$ 1.36	$ (0.40)
Incremental increase in other operating revenues	$ 57	$ 158
Affiliate debt rationalization	34								$ 34
Research and development credits	28								28
Changes affecting state income taxes	$ 16								$ (16)	$ (13)
Loss on worthless investment in foreign subsidiary					$ 60
Settlements					$ (63)