Unaudited Quarterly Information (Tables)

12 Months Ended

Dec. 31, 2017

Unaudited Quarterly Information [Abstract]

Schedule of Quarterly Financial Information [Table Text Block]

Summarized quarterly financial data is as follows:

	Year Ended December 31, 2017
	First Quarter		Second Quarter		Third Quarter		Fourth Quarter
	(in millions)
Revenues	$	638	$	752	$	843	$	765
Operating income	77		164		247		123
Net income (1)	18		75		130		210

	Year Ended December 31, 2016
	First Quarter		Second Quarter		Third Quarter		Fourth Quarter
	(in millions)
Revenues	$	656	$	763	$	909	$	731
Operating income	79		158		258		132
Net income	17		71		136		52

(1) Net income for the fourth quarter 2017 includes a reduction in income taxes of $158 million due to tax reform. See Note 9 for further discussion of the impacts of tax reform implementation.