Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Apr. 21, 2017
Other Commitments [Line Items]
Operating leases, future minimum payments due	$ 75,365	$ 75,365
Land [Member]
Other Commitments [Line Items]
Operating lease, expense		13,300	$ 10,200
Operating Lease, Corporate Headquarters, San Francisco, CA [Member]
Other Commitments [Line Items]
Operating leases, future minimum payments due	35,000	35,000
Power Sale Agreements [Member]
Other Commitments [Line Items]
Letters of credit issued	156,400	$ 156,400
Power Sale Agreements [Member] | Minimum [Member]
Other Commitments [Line Items]
Termination date		2019
Power Sale Agreements [Member] | Maximum [Member]
Other Commitments [Line Items]
Termination date		2043
Project Finance Agreements [Member]
Other Commitments [Line Items]
Letters of credit issued	196,900	$ 196,900
Turbine Manufacturer [Member]
Other Commitments [Line Items]
Loss contingency accrual	300	300
Broadview Project [Member]
Other Commitments [Line Items]
Contingent liability	30,700	30,700
Contingent liabilities, current	1,100	1,100
Contingent liabilities, noncurrent	29,600	29,600
Broadview Project [Member] | Continued Operation Of Broadview [Member]
Other Commitments [Line Items]
Contingent liability				$ 7,200
Broadview Project [Member] | Continued Operation Of Grady [Member]
Other Commitments [Line Items]
Contingent liability					$ 29,000
Panhandle Two [Member]
Other Commitments [Line Items]
Loss contingency accrual	3,700	$ 3,700
Panhandle Two [Member] | Maximum [Member]
Other Commitments [Line Items]
Loss contingency accrual				$ 5,000
Gulf Wind Re-Powering Commitment [Member]
Other Commitments [Line Items]
Maximum purchase price	$ 150,600