Summary of Significant Accounting Policies - Summary of valuation techniques and observable inputs can vary by investment (Detail) - USD ($) $ in Thousands |
Sep. 30, 2025 |
Dec. 31, 2024 |
Dec. 31, 2023 |
|---|---|---|---|
| Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] | |||
| Investment in ETC | $ 203,190 | $ 283,055 | $ 253,958 |
| Level 1 | |||
| Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] | |||
| Investment in ETC | 203,190 | 283,055 | |
| Level 2 | |||
| Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] | |||
| Investment in ETC | 0 | 0 | |
| Level 3 | |||
| Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] | |||
| Investment in ETC | $ 0 | $ 0 |
| X | ||||||||||
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. Reference 1: http://www.xbrl.org/2003/role/exampleRef
|
| X | ||||||||||
- Definition Fair value of investment in security owned. Reference 1: http://www.xbrl.org/2003/role/disclosureRef
|
| X | ||||||||||
- Details
|
| X | ||||||||||
- Details
|
| X | ||||||||||
- Details
|