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)
2017
Operating revenues	$	2,162	2,133	2,141	2,114		8,550
Operating income	580		573	560	600		2,313
Income tax expense (benefit)	174		170	168	(378	)	134
Net income	278		268	265	846		1,657

	First Quarter		Second Quarter	Third Quarter	Fourth Quarter	Total
	(Dollars in millions)
2016
Operating revenues	$	2,253	2,223	2,226	2,208	8,910
Operating income	625		602	576	521	2,324
Income tax expense	188		179	159	152	678
Net income	304		288	255	238	1,085