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Goodwill and Other Intangible Assets - Schedule of Acquisition Date Fair Values, Accumulated Amortization and Net Carrying Values for Other Intangibles Assets and Goodwill, By Company (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Dec. 31, 2020 | Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2019
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Goodwill, fair value | $ 659.6 |  | $ 659.6 |  | $ 525.5 |  |  |
Goodwill, impairment loss |  |  | 30.2 |  | 0.0 |  |  |
Goodwill, net carrying value | 629.4 |  | 629.4 |  | 525.5 |  |  |
Other intangible assets, acquisition date fair value | 493.8 |  | 493.8 |  | 317.8 |  |  |
Intangible assets and goodwill, fair value | 1,153.4 |  | 1,153.4 |  | 843.3 |  |  |
Other intangible assets, accumulated amortization | 72.8 |  | 72.8 |  | 54.7 |  |  |
Other intangible assets, impairments |  |  | 6.2 |  | 6.2 |  |  |
Impairment of intangible assets, including goodwill |  |  | 36.4 |  | 6.2 |  |  |
Other intangible assets, net carrying value | 414.8 |  | 414.8 |  | 256.9 |  |  |
Goodwill and other intangible assets | 1,044.2 |  | 1,044.2 |  | 782.4 |  |  |
Goodwill |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Goodwill, impairment loss |  |  | 30.2 | $ 0.0 |  |  |  |
Goodwill, net carrying value | 629.4 | $ 518.2 | 629.4 | 518.2 | 525.5 | $ 613.4 | $ 392.4 | $ 394.7
Goodwill, foreign currency translation gain (loss) | 0.2 | 0.7 | 1.7 | (1.8) |  |  |  |
Other Intangible Assets |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Goodwill, impairment loss |  |  | 0.0 | 0.0 |  |  |  |
Other intangible assets, foreign currency translation gain (loss) | (0.1) | $ 0.1 | 0.1 | $ (0.4) |  |  |  |
Non-controlling interest |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Goodwill and other intangible assets | 115.9 |  | 115.9 |  | 28.1 |  |  |
Total |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Goodwill and other intangible assets | 928.3 |  | 928.3 |  | 754.3 |  |  |
Ark |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Goodwill, fair value | 116.8 |  | 116.8 |  | 0.0 |  |  |
Goodwill, impairment loss |  |  | 0.0 |  | 0.0 |  |  |
Goodwill, net carrying value | 116.8 |  | 116.8 |  | 0.0 |  |  |
Ark | Trade names |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Other intangible assets, acquisition date fair value | 175.7 |  | 175.7 |  | 0.0 |  |  |
Other intangible assets, accumulated amortization | 0.0 |  | 0.0 |  | 0.0 |  |  |
Other intangible assets, impairments |  |  | 0.0 |  | 0.0 |  |  |
Other intangible assets, net carrying value | 175.7 |  | 175.7 |  | 0.0 |  |  |
Specialty Insurance Distribution (NSM) |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Goodwill, fair value | 508.1 |  | 508.1 |  | 506.4 |  |  |
Goodwill, impairment loss |  |  | 30.2 |  | 0.0 |  |  |
Goodwill, net carrying value | 477.9 |  | 477.9 |  | 506.4 |  |  |
Other intangible assets, acquisition date fair value | 289.2 |  | 289.2 |  | 288.9 |  |  |
Other intangible assets, accumulated amortization | 69.3 |  | 69.3 |  | 52.3 |  |  |
Other intangible assets, impairments |  |  | 6.2 |  | 6.2 |  |  |
Other intangible assets, net carrying value | 213.7 |  | 213.7 |  | 230.4 |  |  |
Specialty Insurance Distribution (NSM) | Goodwill |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Goodwill, foreign currency translation gain (loss) |  |  | 1.5 |  | 13.4 |  |  |
Specialty Insurance Distribution (NSM) | Other Intangible Assets |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Other intangible assets, foreign currency translation gain (loss) |  |  | $ 0.2 |  | 1.6 |  |  |
Specialty Insurance Distribution (NSM) | Customer relationships |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Other intangible assets, weighted average useful life (in years) |  |  | 8 years 10 months 24 days |  |  |  |  |
Other intangible assets, acquisition date fair value | 136.4 |  | $ 136.4 |  | 136.2 |  |  |
Other intangible assets, accumulated amortization | 46.3 |  | 46.3 |  | 36.7 |  |  |
Other intangible assets, impairments |  |  | 3.5 |  | 3.5 |  |  |
Other intangible assets, net carrying value | 86.6 |  | $ 86.6 |  | 96.0 |  |  |
Specialty Insurance Distribution (NSM) | Trade names |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Other intangible assets, weighted average useful life (in years) |  |  | 16 years |  |  |  |  |
Other intangible assets, acquisition date fair value | 65.5 |  | $ 65.5 |  | 65.4 |  |  |
Other intangible assets, accumulated amortization | 10.0 |  | 10.0 |  | 8.3 |  |  |
Other intangible assets, impairments |  |  | 1.0 |  | 1.0 |  |  |
Other intangible assets, net carrying value | 54.5 |  | 54.5 |  | 56.1 |  |  |
Specialty Insurance Distribution (NSM) | Information technology platform |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Other intangible assets, acquisition date fair value | 3.1 |  | 3.1 |  | 3.1 |  |  |
Other intangible assets, accumulated amortization | 1.4 |  | 1.4 |  | 1.4 |  |  |
Other intangible assets, impairments |  |  | 1.7 |  | 1.7 |  |  |
Other intangible assets, net carrying value | 0.0 |  | $ 0.0 |  | 0.0 |  |  |
Specialty Insurance Distribution (NSM) | Renewal rights |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Other intangible assets, weighted average useful life (in years) |  |  | 12 years |  |  |  |  |
Other intangible assets, acquisition date fair value | 82.5 |  | $ 82.5 |  | 82.5 |  |  |
Other intangible assets, accumulated amortization | 10.4 |  | 10.4 |  | 4.9 |  |  |
Other intangible assets, impairments |  |  | 0.0 |  | 0.0 |  |  |
Other intangible assets, net carrying value | 72.1 |  | $ 72.1 |  | 77.6 |  |  |
Specialty Insurance Distribution (NSM) | Other Intangible Assets |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Other intangible assets, weighted average useful life (in years) |  |  | 3 years 4 months 24 days |  |  |  |  |
Other intangible assets, acquisition date fair value | 1.7 |  | $ 1.7 |  | 1.7 |  |  |
Other intangible assets, accumulated amortization | 1.2 |  | 1.2 |  | 1.0 |  |  |
Other intangible assets, impairments |  |  | 0.0 |  | 0.0 |  |  |
Other intangible assets, net carrying value | 0.5 |  | 0.5 |  | 0.7 |  |  |
Kudu |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Goodwill, fair value | 7.6 |  | 7.6 |  | 7.6 |  |  |
Goodwill, impairment loss |  |  | 0.0 |  | 0.0 |  |  |
Goodwill, net carrying value | 7.6 |  | $ 7.6 |  | 7.6 |  |  |
Kudu | Trade names |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Other intangible assets, weighted average useful life (in years) |  |  | 7 years |  |  |  |  |
Other intangible assets, acquisition date fair value | 2.2 |  | $ 2.2 |  | 2.2 |  |  |
Other intangible assets, accumulated amortization | 0.7 |  | 0.7 |  | 0.6 |  |  |
Other intangible assets, impairments |  |  | 0.0 |  | 0.0 |  |  |
Other intangible assets, net carrying value | 1.5 |  | 1.5 |  | 1.6 |  |  |
Other Operations |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Goodwill, fair value | 27.1 |  | 27.1 |  | 11.5 |  |  |
Goodwill, impairment loss |  |  | 0.0 |  | 0.0 |  |  |
Goodwill, net carrying value | 27.1 |  | 27.1 |  | 11.5 |  |  |
Other intangible assets, acquisition date fair value | 26.7 |  | 26.7 |  | 26.7 |  |  |
Other intangible assets, accumulated amortization | 2.8 |  | 2.8 |  | 1.8 |  |  |
Other intangible assets, impairments |  |  | 0.0 |  | 0.0 |  |  |
Other intangible assets, net carrying value | 23.9 |  | $ 23.9 |  | 24.9 |  |  |
Other Operations | Customer relationships |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Other intangible assets, weighted average useful life (in years) |  |  | 10 years 8 months 12 days |  |  |  |  |
Other intangible assets, acquisition date fair value | 14.2 |  | $ 14.2 |  | 14.2 |  |  |
Other intangible assets, accumulated amortization | 2.1 |  | 2.1 |  | 1.4 |  |  |
Other intangible assets, impairments |  |  | 0.0 |  | 0.0 |  |  |
Other intangible assets, net carrying value | 12.1 |  | $ 12.1 |  | 12.8 |  |  |
Other Operations | Trade names |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Other intangible assets, weighted average useful life (in years) |  |  | 9 years 7 months 6 days |  |  |  |  |
Other intangible assets, acquisition date fair value | 3.6 |  | $ 3.6 |  | 3.6 |  |  |
Other intangible assets, accumulated amortization | 0.5 |  | 0.5 |  | 0.3 |  |  |
Other intangible assets, impairments |  |  | 0.0 |  | 0.0 |  |  |
Other intangible assets, net carrying value | 3.1 |  | $ 3.1 |  | 3.3 |  |  |
Other Operations | Other Intangible Assets |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Other intangible assets, weighted average useful life (in years) |  |  | 5 years 4 months 24 days |  |  |  |  |
Other intangible assets, acquisition date fair value | 0.3 |  | $ 0.3 |  | 0.3 |  |  |
Other intangible assets, accumulated amortization | 0.2 |  | 0.2 |  | 0.1 |  |  |
Other intangible assets, impairments |  |  | 0.0 |  | 0.0 |  |  |
Other intangible assets, net carrying value | 0.1 |  | 0.1 |  | 0.2 |  |  |
Other Operations | Insurance Licenses |  |  |  |  |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |  |  |  |  |
Other intangible assets, acquisition date fair value | 8.6 |  | 8.6 |  | 8.6 |  |  |
Other intangible assets, accumulated amortization | 0.0 |  | 0.0 |  | 0.0 |  |  |
Other intangible assets, impairments |  |  | 0.0 |  | 0.0 |  |  |
Other intangible assets, net carrying value | $ 8.6 |  | $ 8.6 |  | $ 8.6 |  |  |

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