Quarterly Results (unaudited) (Tables)

12 Months Ended

Dec. 31, 2016

Quarterly Financial Information Disclosure [Abstract]

Schedule of quarterly results

($ in millions)	First Quarter				Second Quarter				Third Quarter				Fourth Quarter
	2016		2015		2016		2015		2016		2015		2016		2015
Revenues	$	685	$	927	$	742	$	862	$	713	$	992	$	751	$	641
Net income	52		153		94		150		65		239		108		19