Debt - Additional Information (Details) - USD ($)
 1 Months Ended
Jul. 31, 2019
Nov. 21, 2017
Sep. 30, 2019
Aug. 31, 2018
Mar. 31, 2018
Dec. 31, 2018
Jun. 30, 2018
Jul. 31, 2015
Line of Credit Facility [Line Items]                
Long-term debt, gross     $ 2,590,000,000     $ 2,315,000,000    
Convertible debt                
Line of Credit Facility [Line Items]                
Principal     $ 225,000,000     225,000,000   $ 225,000,000
Contractual interest rate               4.00%
Gulf Wind Promissory Note                
Line of Credit Facility [Line Items]                
Contractual interest rate     2.84%          
Long-term debt, gross     $ 22,000,000     0    
Tsugaru Construction Loan                
Line of Credit Facility [Line Items]                
Contractual interest rate     0.72%          
Long-term debt, gross     $ 276,000,000     131,000,000    
Tsugaru Construction Loan | Tokyo Interbank Offered Rate (TIBOR)                
Line of Credit Facility [Line Items]                
Basis spread on variable rate         0.65%      
Japanese Consumption Tax Facility                
Line of Credit Facility [Line Items]                
Commitment fee percentage         0.30%      
Tsugaru Holdings Loan Agreement                
Line of Credit Facility [Line Items]                
Maximum borrowing capacity         $ 70,000,000      
Contractual interest rate     3.13%          
Commitment fee percentage         0.50%      
Long-term debt, gross     $ 62,000,000     59,000,000    
Tsugaru Holdings Loan Agreement | Tokyo Interbank Offered Rate (TIBOR)                
Line of Credit Facility [Line Items]                
Basis spread on variable rate         3.00%      
Revolving Credit Facility                
Line of Credit Facility [Line Items]                
Maximum borrowing capacity   $ 440,000,000 440,000,000          
Debt term   5 years            
Remaining borrowing capacity     347,000,000          
Letters of credit issued     54,000,000     $ 45,000,000    
Incremental Bank Loan                
Line of Credit Facility [Line Items]                
Debt term 3 years              
Line of credit term loan $ 250,000,000              
Incremental Bank Loan | Federal Funds Rate                
Line of Credit Facility [Line Items]                
Basis spread on variable rate 0.50%              
Incremental Bank Loan | Eurodollar                
Line of Credit Facility [Line Items]                
Interest rate 1.00%              
Incremental Bank Loan | Minimum | Eurodollar                
Line of Credit Facility [Line Items]                
Basis spread on variable rate 1.175%              
Incremental Bank Loan | Minimum | Base Rate                
Line of Credit Facility [Line Items]                
Basis spread on variable rate 0.175%              
Incremental Bank Loan | Maximum | Eurodollar                
Line of Credit Facility [Line Items]                
Basis spread on variable rate 1.425%              
Incremental Bank Loan | Maximum | Base Rate                
Line of Credit Facility [Line Items]                
Basis spread on variable rate 0.425%              
Japan Credit Facility                
Line of Credit Facility [Line Items]                
Maximum borrowing capacity       $ 33,000,000        
Remaining borrowing capacity     8,000,000          
Commitment fee percentage       0.30%        
Japan Credit Facility | Minimum | Tokyo Interbank Offered Rate (TIBOR)                
Line of Credit Facility [Line Items]                
Basis spread on variable rate       1.75%        
Japan Credit Facility | Maximum | Tokyo Interbank Offered Rate (TIBOR)                
Line of Credit Facility [Line Items]                
Basis spread on variable rate       2.25%        
Tsugaru Credit Facility | Tsugaru Construction Loan                
Line of Credit Facility [Line Items]                
Maximum borrowing capacity         $ 371,000,000      
Debt term, following conversion         18 years      
Tsugaru Credit Facility | Debt Service Reserve Account LC                
Line of Credit Facility [Line Items]                
Principal         $ 20,000,000      
Tsugaru Credit Facility | O&M Reserve Account LC                
Line of Credit Facility [Line Items]                
Principal         $ 8,000,000      
Tsugaru Credit Facility | Tsugaru Letter Of Credit Facilities                
Line of Credit Facility [Line Items]                
Contractual interest rate             1.10%  
Unused capacity, commitment fee percentage         0.30%      
Tsugaru Credit Facility | Japanese Consumption Tax Facility                
Line of Credit Facility [Line Items]                
Maximum borrowing capacity         $ 34,000,000      
Tsugaru Credit Facility | Japanese Consumption Tax Facility | Tokyo Interbank Offered Rate (TIBOR)                
Line of Credit Facility [Line Items]                
Basis spread on variable rate         0.30%      
Bridge Loan | Gulf Wind Promissory Note                
Line of Credit Facility [Line Items]                
Principal     $ 22,000,000          
Bridge Loan | Gulf Wind Promissory Note | LIBOR                
Line of Credit Facility [Line Items]                
Basis spread on variable rate     0.75%