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Summary of Significant Accounting Principles and Practices - Additional Information (Details) £ in Millions, $ in Millions | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2022 USD ($) | Dec. 31, 2022 GBP (£) | Dec. 31, 2021 USD ($) | Dec. 31, 2021 GBP (£) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Accounting Policies [Abstract] |  |  |  |  |  |
Cash and cash equivalents and short-term investments | $ 1,142.0 |  | $ 836.0 |  |  |
Cash and cash equivalents and short term investments, period increase (decrease) | 306.0 |  |  |  |  |
Restricted cash and investments, current | 115.0 |  | 160.0 |  |  |
Operating funds required to be held by the Company in the U.K. | 72.5 | £ 60.1 | 112.8 | £ 84.3 |  |
Premium trust balances | 6,400.0 |  | 6,100.0 |  |  |
Fiduciary receivables | 9,500.0 |  | 8,300.0 |  |  |
Allowance for doubtful accounts | $ 76.0 |  | $ 90.0 |  | $ 98.0 | $ 77.0

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- Definition Cash And Cash Equivalents And Short Term Investments, Period Increase (Decrease) + References No definition available. + Details
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- Definition Amount of allowance for credit loss on accounts receivable, classified as current. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI https://asc.fasb.org/extlink&oid=123577603&loc=d3e5074-111524 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 45 -Paragraph 1 -URI https://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(4)) -URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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- Definition The current cash, cash equivalents and investments that are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against short-term borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. Includes current cash equivalents and investments that are similarly restricted as to withdrawal, usage or disposal. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 954 -SubTopic 210 -Section 45 -Paragraph 4 -URI https://asc.fasb.org/extlink&oid=120413173&loc=SL6242262-115580 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(9)) -URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 954 -SubTopic 210 -Section 50 -Paragraph 2 -URI https://asc.fasb.org/extlink&oid=127002003&loc=SL6242269-115581 + Details
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