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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 |  |  |  |

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