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Description of Organization and Business Operations - Additional Information (Details) - USD ($)
 3 Months Ended 12 Months Ended
Dec. 03, 2020
Aug. 26, 2020
Aug. 20, 2020
Mar. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Subsidiary, Sale of Stock [Line Items]            
Common stock, par value       $ 0.000001 $ 0.000001 $ 0.000001
Cash       $ 9,873,000 $ 6,942,000 $ 12,889,000
Star Peak Energy Transition Corp [Member]            
Subsidiary, Sale of Stock [Line Items]            
Share price per share         $ 10.00  
Other offering costs         $ 7,904,149 24,500
Investment of Cash into Trust Account     $ 350,000,000.0   $ 383,585,040  
Threshold minimum aggregate fair market value as a percentage of the assets held in the Trust Account         80.00%  
Threshold percentage of outstanding voting securities of the target to be acquired by post-transaction company to complete business combination       50.00% 50.00%  
Minimum target assets of the target         $ 5,000,001  
Stock redemption restriction percentage requiring company consent         15.00%  
Obligation to redeem Public Shares if entity does not complete a Business Combination (as a percent)         100.00%  
Threshold business days for redemption of public shares         10 days  
Maximum net interest to pay dissolution expenses         $ 100,000  
Cash       $ 426,064 936,773  
Interest income available in the Trust Account       35,469 136,707  
Working capital       1,700,000 1,200,000  
Aggregate purchase price         376,932,978  
Loan from the Sponsor         200,000  
Loans from working capital       $ 0 $ 0 $ 0
Star Peak Energy Transition Corp [Member] | Class A shares            
Subsidiary, Sale of Stock [Line Items]            
Common stock, par value       $ 0.0001 $ 0.0001  
Interest income available in the Trust Account       $ 35,000 $ 137,000  
Star Peak Energy Transition Corp [Member] | Sponsor | Founder Shares            
Subsidiary, Sale of Stock [Line Items]            
Aggregate purchase price       25,000 25,000  
Loan from the Sponsor       $ 300,000 $ 300,000  
Star Peak Energy Transition Corp [Member] | Agreement and Plan of Merger | New Stem Common Stock            
Subsidiary, Sale of Stock [Line Items]            
Pro rata portion of shares issuable as right to receive upon conversion (in shares) 65,000,000          
Number of shares expected to be issued (in shares) 65,000,000          
Star Peak Energy Transition Corp [Member] | Agreement and Plan of Merger | Stem common stock holders | New Stem Common Stock            
Subsidiary, Sale of Stock [Line Items]            
Percentage of shares issued to be held by stock holders (as a percent) 48.00%          
Percentage of shares outstanding to be held by stock holders (as a percent) 66.90%          
Star Peak Energy Transition Corp [Member] | Initial Public Offering            
Subsidiary, Sale of Stock [Line Items]            
Number of units issued   3,358,504 35,000,000      
Share price per share   $ 10.00 $ 10.00 $ 10.00    
Gross proceeds from sale of units     $ 350,000,000.0      
Other offering costs     20,200,000      
Deferred underwriting fee payable     $ 12,300,000      
Star Peak Energy Transition Corp [Member] | Over-allotment            
Subsidiary, Sale of Stock [Line Items]            
Number of units issued   3,358,504   5,250,000 5,250,000  
Share price per share   $ 10.00   $ 10.00 $ 10.00  
Gross proceeds from sale of units   $ 33,600,000        
Other offering costs   1,900,000        
Deferred underwriting fee payable   1,200,000        
Investment of Cash into Trust Account   $ 34,300,000        
Star Peak Energy Transition Corp [Member] | Private Placement            
Subsidiary, Sale of Stock [Line Items]            
Number of warrants issued         6,733,333  
Warrant Issue Price       $ 1.50 $ 1.50  
Proceeds from sale of Private Placement Warrants       $ 10,100,000 $ 10,100,000  
Aggregate purchase price       $ 1.50 $ 1.50  
Star Peak Energy Transition Corp [Member] | Private Placement | Sponsor            
Subsidiary, Sale of Stock [Line Items]            
Number of warrants issued   447,801        
Proceeds from sale of Private Placement Warrants   $ 700,000        
Star Peak Energy Transition Corp [Member] | As Previously Reported | Over-allotment            
Subsidiary, Sale of Stock [Line Items]            
Investment of Cash into Trust Account   $ 33,600,000