Quarterly report pursuant to Section 13 or 15(d)

The Company - Additional Information (Detail)

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The Company - Additional Information (Detail)
 1 Months Ended 9 Months Ended
Jun. 30, 2015
USD ($)
May. 31, 2014
USD ($)
Sep. 30, 2015
Companies
Project
May. 31, 2015
USD ($)
Oct. 31, 2014
USD ($)
Organization [Line Items]          
Acquisition date     May 28, 2014    
Business combination consideration paid   $ 107,000,000      
Acquisition related costs   2,500,000      
Liabilities assumed in connection with acquisitions   0      
Acquisition completed date     Jan. 01, 2013    
JPMorgan [Member]          
Organization [Line Items]          
Proceeds from joint ventures $ 3,000,000        
Wind Projects [Member]          
Organization [Line Items]          
Number of limited liability companies | Companies     5    
Number of operating projects | Project     10    
Number of projects under construction | Project     1    
Strong Upwind Holdings I, LLC [Member] | JPMorgan [Member]          
Organization [Line Items]          
Project investment amount $ 141,000,000     $ 144,000,000  
Strong Upwind Holdings I, LLC [Member] | Wind Projects [Member] | JPMorgan [Member]          
Organization [Line Items]          
Project investment amount         $ 141,000,000
AWCC [Member]          
Organization [Line Items]          
Business combination consideration paid   $ 19,000,000