Quarterly Results (unaudited) (Tables)

12 Months Ended

Dec. 31, 2015

Quarterly Financial Information Disclosure [Abstract]

Schedule of quarterly results

($ in millions)	First Quarter				Second Quarter				Third Quarter				Fourth Quarter
	2015		2014		2015		2014		2015		2014		2015		2014
Revenues	$	927	$	1,049	$	862	$	851	$	992	$	823	$	641	$	937
Net income	153		127		150		132		239		84		19		183