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Significant Transactions - NSM (Details) £ in Millions, $ in Millions |  |  |  |  |  | 3 Months Ended | 6 Months Ended | 9 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Apr. 12, 2021 USD ($) | Apr. 12, 2021 GBP (£) | Apr. 07, 2020 USD ($) | Apr. 07, 2020 GBP (£) | Mar. 31, 2019 USD ($) | Dec. 03, 2018 USD ($) | May 18, 2018 USD ($) earnout_period | Jun. 30, 2021 USD ($) | Mar. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Jun. 30, 2020 USD ($) | Mar. 31, 2020 USD ($) | Jun. 30, 2021 USD ($) | Jun. 30, 2020 USD ($) | Sep. 30, 2019 USD ($) | Apr. 07, 2020 GBP (£)
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss on sale |  |  |  |  |  |  |  |  | $ 28.7 |  |  |  |  |  |  |
Goodwill |  |  |  |  |  |  |  | $ 629.4 |  | $ 525.5 |  |  | $ 629.4 |  |  |
Change in fair value of contingent consideration earnout liabilities |  |  |  |  |  |  |  | 0.2 |  |  | $ (1.7) |  | 0.2 | $ (2.3) |  |
Contingent consideration payments related to acquisitions of subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  | 7.5 | 6.4 |  |
Recognized pre-tax expense (revenue) |  |  |  |  |  |  |  | 0.2 |  |  | $ (1.7) |  | 0.2 | $ (2.3) |  |
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Fresh Insurance Services Group Limited |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sale of business | $ 1.5 | £ 1.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disposal Group, Held-for-sale, Not Discontinued Operations | Fresh Insurance Services Group Limited |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss on sale |  |  |  |  |  |  |  |  | 28.7 |  |  |  |  |  |  |
Fresh Insurance Services Group Limited |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |
Equity method investment, aggregate cost |  |  |  |  |  |  | $ 49.6 |  |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  |  | $ 51.0 |  |  |  |  |  |  |  |  |
Purchase price adjustment |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.7 |
Number of earnout periods | earnout_period |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |
KBK Insurance Group, Inc. & KBK Premium Services, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Borrowings |  |  |  |  |  | $ 30.1 |  |  |  |  |  |  |  |  |  |
Cash consideration to acquire businesses and interest in affiliates |  |  |  |  |  | 60.0 |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  |  | $ 32.6 |  |  |  |  |  |  |  |  |  |  |
Intangible assets acquired |  |  |  |  | 32.7 |  |  |  |  |  |  |  |  |  |  |
NSM Insurance HoldCo, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash consideration to acquire businesses and interest in affiliates |  |  |  |  |  | $ 29.0 |  |  |  |  |  |  |  |  |  |
Specialty Insurance Distribution (NSM) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contingent consideration |  |  |  |  |  |  |  | 7.3 |  | 14.6 |  |  | 7.3 |  |  |
Specialty Insurance Distribution (NSM) | Kingsbridge Group Limited |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment, aggregate cost |  |  | $ 132.2 |  |  |  |  |  |  |  |  |  |  |  |  | £ 107.2
Borrowings |  |  | $ 52.4 | £ 42.5 |  |  |  |  |  |  |  |  |  |  |  |
Percentage of voting interests acquired |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  | 100.00%
Stock issued during period, value, acquisitions |  |  | $ 80.3 |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions of businesses |  |  | 111.5 |  |  |  |  |  |  |  |  |  |  |  |  |
Attribution of acquisition date fair value estimates between goodwill and other intangible assets |  |  | 20.2 |  |  |  |  |  |  |  |  |  |  |  |  |
Contingent consideration |  |  | $ 4.1 |  |  |  |  | 0.1 |  |  |  |  | 0.1 |  |  |
Pretax income |  |  |  |  |  |  |  |  |  | 4.1 |  |  |  |  |  |
Foreign currency translation gain |  |  |  |  |  |  |  |  |  | $ 0.3 |  |  |  |  |  |
KBK Insurance Group, Inc. & KBK Premium Services, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Change in fair value of contingent consideration earnout liabilities |  |  |  |  | $ 5.9 |  |  |  |  |  |  |  |  |  |  |
Contingent consideration payments related to acquisitions of subsidiaries |  |  |  |  |  |  |  |  | $ 6.7 |  |  | $ 6.4 |  |  |  |
KBK Insurance Group, Inc. & KBK Premium Services, Inc. | Specialty Insurance Distribution (NSM) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contingent consideration |  |  |  |  |  |  |  | $ 6.6 |  |  |  |  | $ 6.6 |  |  |

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