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Equity Method Investments in Affiliates - Additional Information (Details) shares in Millions, $ in Millions |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Nov. 03, 2025 USD ($) | Jul. 31, 2025 USD ($) shares | Sep. 30, 2025 USD ($) affiliate | Sep. 30, 2024 USD ($) | Jun. 30, 2024 USD ($) yr | Sep. 30, 2025 USD ($) affiliate | Sep. 30, 2024 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2024 USD ($) affiliate
Estimated Future Amortization Expense of Finite-Lived Intangibles |  |  |  |  |  |  |  |  |
Number of affiliates | affiliate |  |  | 23 |  |  | 23 |  |  | 22
Affiliate transaction gains |  |  | $ 127.6 | $ 0.0 |  | $ 127.6 | $ 0.0 |  |
Equity method investments |  |  | 118.3 |  |  | 118.3 |  |  | $ 111.4
Peppertree Capital Management | TPG Inc. |  |  |  |  |  |  |  |  |
Estimated Future Amortization Expense of Finite-Lived Intangibles |  |  |  |  |  |  |  |  |
Sale of equity interests, total consideration |  | $ 253.2 |  |  |  |  |  |  |
Sale of equity interests, cash proceeds |  | $ 99.8 |  |  |  |  |  |  |
Sale of equity interests, equity interests issuable, number of shares (in shares) | shares |  | 2.9 |  |  |  |  |  |  |
Affiliate transaction gains |  | $ 127.6 |  |  |  |  |  |  |
Sale of equity, after-tax net proceeds |  | $ 218.1 |  |  |  |  |  |  |
Comvest Partners |  |  |  |  |  |  |  |  |
Estimated Future Amortization Expense of Finite-Lived Intangibles |  |  |  |  |  |  |  |  |
Acquisition of equity method investments |  |  |  |  |  |  |  | $ 125.0 |
Equity method investments |  |  | 121.8 |  |  | 121.8 |  |  |
Comvest Partners | Subsequent Event |  |  |  |  |  |  |  |  |
Estimated Future Amortization Expense of Finite-Lived Intangibles |  |  |  |  |  |  |  |  |
Proceeds from sale of equity method investments | $ 285.0 |  |  |  |  |  |  |  |
Acquired Client Relationships |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Amortization of intangible assets |  |  | 6.3 | 7.3 |  | 19.0 | 21.8 |  |
Estimated Future Amortization Expense of Finite-Lived Intangibles |  |  |  |  |  |  |  |  |
Finite-lived intangible, future amortization expense, remainder of fiscal year |  |  | 6.0 |  |  | 6.0 |  |  |
Finite-lived intangible, future amortization expense in year one |  |  | 25.0 |  |  | 25.0 |  |  |
Finite-lived intangible, future amortization expense in year two |  |  | 25.0 |  |  | 25.0 |  |  |
Finite-lived intangible, future amortization expense in year three |  |  | 25.0 |  |  | 25.0 |  |  |
Finite-lived intangible, future amortization expense in year four |  |  | 15.0 |  |  | 15.0 |  |  |
Finite-lived intangible, future amortization expense in year five |  |  | 10.0 |  |  | 10.0 |  |  |
Equity Method Investee |  |  |  |  |  |  |  |  |
Estimated Future Amortization Expense of Finite-Lived Intangibles |  |  |  |  |  |  |  |  |
Impairment expense |  |  |  |  | $ 39.9 |  |  |  |
Equity Method Investee | Measurement Input, Growth Rate Period |  |  |  |  |  |  |  |  |
Estimated Future Amortization Expense of Finite-Lived Intangibles |  |  |  |  |  |  |  |  |
Equity method investment, measurement input | yr |  |  |  |  | 10 |  |  |  |
Equity Method Investee | Measurement Input, Growth Rate |  |  |  |  |  |  |  |  |
Estimated Future Amortization Expense of Finite-Lived Intangibles |  |  |  |  |  |  |  |  |
Equity method investment, measurement input |  |  |  |  | (0.025) |  |  |  |
Equity Method Investee | Measurement Input, Long-Term Growth Rate |  |  |  |  |  |  |  |  |
Estimated Future Amortization Expense of Finite-Lived Intangibles |  |  |  |  |  |  |  |  |
Equity method investment, measurement input |  |  |  |  | 0.03 |  |  |  |
Equity Method Investee | Measurement Input, Discount Rate, Asset Based Fees |  |  |  |  |  |  |  |  |
Estimated Future Amortization Expense of Finite-Lived Intangibles |  |  |  |  |  |  |  |  |
Equity method investment, measurement input |  |  |  |  | 0.12 |  |  |  |
Equity Method Investee | Measurement Input, Discount Rate, Performance Based Fees |  |  |  |  |  |  |  |  |
Estimated Future Amortization Expense of Finite-Lived Intangibles |  |  |  |  |  |  |  |  |
Equity method investment, measurement input |  |  |  |  | 0.20 |  |  |  |
Equity Method Investee | Measurement Input, Market Participant Tax Rates |  |  |  |  |  |  |  |  |
Estimated Future Amortization Expense of Finite-Lived Intangibles |  |  |  |  |  |  |  |  |
Equity method investment, measurement input |  |  |  |  | 0.21 |  |  |  |
Equity Method Investee | Acquired Client Relationships |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Amortization of intangible assets |  |  | 24.6 | $ 22.7 |  | 70.2 | $ 64.4 |  |
Estimated Future Amortization Expense of Finite-Lived Intangibles |  |  |  |  |  |  |  |  |
Finite-lived intangible, future amortization expense, remainder of fiscal year |  |  | 23.0 |  |  | 23.0 |  |  |
Finite-lived intangible, future amortization expense in year one |  |  | 85.0 |  |  | 85.0 |  |  |
Finite-lived intangible, future amortization expense in year two |  |  | 85.0 |  |  | 85.0 |  |  |
Finite-lived intangible, future amortization expense in year three |  |  | 75.0 |  |  | 75.0 |  |  |
Finite-lived intangible, future amortization expense in year four |  |  | 60.0 |  |  | 60.0 |  |  |
Finite-lived intangible, future amortization expense in year five |  |  | $ 60.0 |  |  | $ 60.0 |  |  |

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