Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2017

Fair Value Disclosures [Abstract]

Schedule of Recurring Fair Value Measurements

Recurring Derivative Fair Value Measures
	As of June 30, 2017
(millions)	Level 1		Level 2		Level 3		Total
Assets
Natural gas swaps	$	0	$	1	$	0	$	1
	As of December 31, 2016
(millions)	Level 1		Level 2		Level 3		Total
Assets
Natural gas swaps	$	0	$	17	$	0	$	17