Variable Interest Entities - Additional Information (Detail) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) MW	Jun. 30, 2016 USD ($)
Long Term Contract For Purchase Of Electric Power [Line Items]
Purchased power | $	$ 8	$ 28	$ 15	$ 42
Power Purchase Agreements [Member] | Variable Interest Entity Not Primary Beneficiary [Member]
Long Term Contract For Purchase Of Electric Power [Line Items]
Purchased power | $	$ 4	$ 16	$ 8	$ 29
Minimum [Member]
Long Term Contract For Purchase Of Electric Power [Line Items]
Multiple PPAs range | MW			117
Maximum [Member]
Long Term Contract For Purchase Of Electric Power [Line Items]
Multiple PPAs range | MW			250