Commitments and Contingencies - Additional Information (Detail) $ in Thousands, $ in Millions				6 Months Ended
	Mar. 07, 2018 USD ($)	Jun. 16, 2017 CAD ($) MW	Apr. 21, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Other Commitments [Line Items]
Operating leases, future minimum payments due				$ 41,112
Land [Member]
Other Commitments [Line Items]
Operating lease, expense				8,900	$ 7,200
Operating Lease, Corporate Headquarters, San Francisco, CA [Member]
Other Commitments [Line Items]
Operating leases, future minimum payments due				35,000
Limited Partner Interest In Newly-formed Limited Partnership [Member]
Other Commitments [Line Items]
Ownership percentage		51.00%
Pattern MSM GP Holdings, Inc. [Member]
Other Commitments [Line Items]
Ownership percentage		70.00%
Pattern Development MSM Management ULC [Member]
Other Commitments [Line Items]
Ownership percentage		70.00%
Power Sale Agreements [Member]
Other Commitments [Line Items]
Letters of credit issued				$ 156,200
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				2042
Project Finance Agreements [Member]
Other Commitments [Line Items]
Letters of credit issued				$ 187,000
Turbine Manufacturer [Member]
Other Commitments [Line Items]
Loss contingency accrual				300
Mont Sainte-Marguerite Wind Farm LP [Member]
Other Commitments [Line Items]
Cash Consideration		$ 53.0
Generating capacity | MW		143
Mont Sainte-Marguerite Wind Farm LP [Member] | Limited Partner Interest In Newly-formed Limited Partnership [Member]
Other Commitments [Line Items]
Percentage of ownership		100.00%
Tsugaru Holdings [Member]
Other Commitments [Line Items]
Cash Consideration	$ 176,600
Contingent liability	16,200
Contingent liabilities, current	$ 16,249
Broadview Project [Member]
Other Commitments [Line Items]
Cash Consideration			$ 214,700
Contingent liability				30,300
Contingent liabilities, current				500
Contingent liabilities, noncurrent				29,800
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
Panhandle Two [Member] | Maximum [Member]
Other Commitments [Line Items]
Loss contingency accrual						$ 5,000