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