Tax Equity Financing Arrangements - Additional Information (Detail) - USD ($)	1 Months Ended	6 Months Ended
	Feb. 28, 2017	Jun. 30, 2017	Dec. 31, 2016
Tax Equity Financing Arrangements [Line Items]
Accrued interest average rate		6.70%	5.40%
Assets of variable interest entities (VIEs)		$ 1,278,000,000	$ 1,343,000,000
Liabilities of variable interest entities (VIEs)		190,000,000	244,000,000
Equity method investments of variable interest entities (VIEs)		392,000,000	387,000,000
Upfront cash payments		0
Locust Ridge Wind Farm, LLC [Member]
Tax Equity Financing Arrangements [Line Items]
Acquisition of equity investor's interest	$ 5,000,000
Variable Interest Entity, Primary Beneficiary [Member]
Tax Equity Financing Arrangements [Line Items]
Equity method investments of variable interest entities (VIEs)		$ 155,000,000	$ 161,000,000