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Significant Transactions - NSM (Details) £ in Millions, $ in Millions |  |  |  |  |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Aug. 06, 2021 USD ($) | Apr. 12, 2021 USD ($) | Apr. 12, 2021 GBP (£) | Apr. 07, 2020 USD ($) | Apr. 07, 2020 GBP (£) | Jun. 28, 2019 USD ($) | Apr. 01, 2019 USD ($) | Dec. 03, 2018 USD ($) | May 18, 2018 USD ($) earnout_period | Mar. 31, 2021 USD ($) | Sep. 30, 2019 USD ($) | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Apr. 07, 2020 GBP (£)
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchases of consolidated subsidiaries, net of cash acquired |  |  |  |  |  |  |  |  |  |  |  | $ 39.1 | $ 127.3 | $ 258.0 |
Goodwill and intangible assets acquired |  |  |  |  |  |  |  | $ 59.4 |  |  |  |  |  |  |
Goodwill |  |  |  |  |  |  |  |  |  |  |  | 645.5 | 525.5 |  |
Change in fair value of contingent consideration |  |  |  |  |  |  |  |  |  |  |  | 1.0 | (3.3) | 2.1 |
Contingent consideration payments related to acquisitions of subsidiaries |  |  |  |  |  |  |  |  |  |  |  | 0.0 | 7.0 | 7.6 |
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) |  | 28.7 |  |  |
Fresh Insurance Services Group Limited |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  |  |  | 100.00% |  |  |  |  |  |
Proceeds from 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] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from borrowings |  |  |  |  |  |  |  | 30.1 |  |  |  |  |  |  |
Purchases of consolidated subsidiaries, net of cash acquired |  |  |  |  |  |  |  | 60.0 |  |  |  |  |  |  |
Goodwill |  |  |  |  |  |  |  |  |  |  |  |  |  | 32.6 |
Intangible assets acquired |  |  |  |  |  |  |  |  |  |  |  |  |  | 32.7 |
NSM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchases of consolidated subsidiaries, net of cash acquired |  |  |  |  |  |  |  | $ 29.0 |  |  |  |  |  |  |
Embrace Pet Insurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  |  |  |  | $ 52.2 |  |  |  |  |  |  |  |
Intangible assets acquired |  |  |  |  |  |  | 15.4 |  |  |  |  |  |  |  |
J.C. Taylor Insurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from borrowings | $ 35.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchases of consolidated subsidiaries, net of cash acquired | 49.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill | 28.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets acquired | 27.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets acquired | 60.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill and other intangible assets | 55.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Liabilities acquired | $ 10.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |
NSM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Change in fair value of contingent consideration |  |  |  |  |  |  |  |  |  |  |  | 1.0 | (3.3) |  |
Contingent consideration |  |  |  |  |  |  |  |  |  |  |  | 6.8 | 14.6 |  |
Goodwill and other intangible assets |  |  |  |  |  |  |  |  |  |  |  | 725.4 | 736.8 |  |
NSM | KBK Insurance Group, Inc. & KBK Premium Services, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contingent consideration payments related to acquisitions of subsidiaries |  |  |  |  |  |  |  |  |  |  |  | 8.8 | 7.0 |  |
NSM | Fresh Insurance Services Group Limited |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment, aggregate cost |  |  |  |  |  |  |  |  | $ 49.6 |  |  |  |  |  |
NSM | Embrace Pet Insurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from borrowings |  |  |  |  |  |  | $ 20.4 |  |  |  |  |  |  |  |
Percentage of voting interests acquired |  |  |  |  |  |  | 100.00% |  |  |  |  |  |  |  |
Payments to acquire businesses, gross |  |  |  |  |  |  | $ 71.5 |  |  |  |  |  |  |  |
Additional investment in acquisition |  |  |  |  |  |  | $ 58.2 |  |  |  |  |  |  |  |
NSM | Kingsbridge Group Limited |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment, aggregate cost |  |  |  | $ 132.2 |  |  |  |  |  |  |  |  |  |  | £ 107.2
Proceeds from borrowings |  |  |  | $ 52.4 | £ 42.5 |  |  |  |  |  |  |  |  |  |
Percentage of voting interests acquired |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  | 100.00%
Additional investment in acquisition |  |  |  | $ 80.3 |  |  |  |  |  |  |  |  |  |  |
Acquisitions of businesses |  |  |  |  |  |  |  |  |  |  |  |  | 111.5 |  |
Finite-lived Intangible Assets Acquired |  |  |  |  |  |  |  |  |  |  |  |  | 20.2 |  |
Pretax income |  |  |  |  |  |  |  |  |  |  |  | (0.3) | 4.1 |  |
Contingent consideration |  |  |  | $ 4.1 |  |  |  |  |  |  |  |  | 0.3 |  |
NSM | J.C. Taylor Insurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of voting interests acquired | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
KBK Insurance Group, Inc. & KBK Premium Services, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Change in fair value of contingent consideration |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 5.9 |
Contingent consideration payments related to acquisitions of subsidiaries |  |  |  |  |  |  |  |  |  |  |  | $ 6.7 | $ 6.4 |  |
AIG Collector Car Business |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchases of consolidated subsidiaries, net of cash acquired |  |  |  |  |  | $ 82.5 |  |  |  |  |  |  |  |  |
Intangible assets acquired |  |  |  |  |  | 82.5 |  |  |  |  |  |  |  |  |
Additional investment in acquisition |  |  |  |  |  | 59.1 |  |  |  |  |  |  |  |  |
AIG Collector Car Business | NSM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from borrowings |  |  |  |  |  | $ 22.5 |  |  |  |  |  |  |  |  |

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