Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Schedule of quarterly financial information

	First Quarter		Second Quarter	Third Quarter	Fourth Quarter	Total
	(Dollars in millions, except per share amounts)
2017
Operating revenues	$	4,209	4,090	4,034	5,323	17,656
Operating income	631		367	487	524	2,009
Net income	163		17	92	1,117	1,389
Basic earnings per common share	0.30		0.03	0.17	1.26	2.21
Diluted earnings per common share	0.30		0.03	0.17	1.26	2.21
2016
Operating revenues	$	4,401	4,398	4,382	4,289	17,470
Operating income	688		647	593	405	2,333
Net income	236		196	152	42	626
Basic earnings per common share	0.44		0.36	0.28	0.08	1.16
Diluted earnings per common share	0.44		0.36	0.28	0.08	1.16