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Summary of Significant Accounting Policies - Summary of Changes to Presentation of Statement of Cash Flows (Details) - USD ($) $ in Millions | 9 Months Ended
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Sep. 30, 2023 | Sep. 30, 2022
Net Cash Provided by (Used in) Operating Activities [Abstract] |  |
Net change in accounts receivable, net | $ (684.7) | $ (506.6)
Net change in other current assets | (17.5) | 5.8
Net change in accrued compensation and other accrued liabilities | 361.1 | (27.5)
Net cash provided by operating activities | 1,338.1 | 782.1
Cash flows from financing activities: |  |
Net change in fiduciary assets and liabilities |  | 1,142.9
Net cash provided by (used) by financing activities | $ 1,891.8 | 873.2
Reported 2022 |  |
Net Cash Provided by (Used in) Operating Activities [Abstract] |  |
Net change in premiums and fees receivable |  | (5,776.8)
Net change in premiums payable to underwriting enterprises |  | 6,200.1
Net change in other current assets |  | 64.9
Net change in accrued compensation and other accrued liabilities |  | 126.4
Net cash provided by operating activities |  | 1,925.0
Cash flows from financing activities: |  |
Net cash provided by (used) by financing activities |  | (269.7)
Change |  |
Net Cash Provided by (Used in) Operating Activities [Abstract] |  |
Net change in accounts receivable, net |  | (506.6)
Net change in premiums and fees receivable |  | 5,776.8
Net change in premiums payable to underwriting enterprises |  | (6,200.1)
Net change in other current assets |  | (59.1)
Net change in accrued compensation and other accrued liabilities |  | (153.9)
Net cash provided by operating activities |  | (1,142.9)
Cash flows from financing activities: |  |
Net change in fiduciary assets and liabilities |  | 1,142.9
Net cash provided by (used) by financing activities |  | $ 1,142.9

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