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v3.20.4

Summary of Significant Accounting Principles and Practices - Additional Information (Details) £ in Millions, $ in Millions | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 31, 2020 USD ($) | Dec. 31, 2020 GBP (£) | Jan. 01, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2019 GBP (£) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Cash and Cash Equivalents |  |  |  |  |  |  |
Cash and cash equivalents and short-term investments | $ 1,192.0 |  |  | $ 928.0 |  |  |
Restricted cash and investments, current | 102.0 |  |  | 110.0 |  |  |
Operating funds in U.K. | 60.2 | £ 44.4 |  | 55.5 | £ 42.7 |  |
Fiduciary Assets and Liabilities |  |  |  |  |  |  |
Premium trust balances | 5,700.0 |  |  | 5,200.0 |  |  |
Allowance for Doubtful Accounts |  |  |  |  |  |  |
Allowance for doubtful accounts | 98.0 |  |  | 70.0 |  | $ 62.0 | $ 59.0
New Accounting Pronouncements and Changes in Accounting Principles |  |  |  |  |  |  |
Cumulative adjustment amount to decrease retained earnings | (3,583.0) |  |  | (3,449.0) |  | (4,219.0) | (4,648.0)
Retained Earnings |  |  |  |  |  |  |
New Accounting Pronouncements and Changes in Accounting Principles |  |  |  |  |  |  |
Cumulative adjustment amount to decrease retained earnings | $ (1,042.0) |  |  | (1,254.0) |  | (2,093.0) | (2,302.0)
Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |
Allowance for Doubtful Accounts |  |  |  |  |  |  |
Allowance for doubtful accounts |  |  | $ 7.0 | 7.0 |  | $ 0.0 | 0.0
New Accounting Pronouncements and Changes in Accounting Principles |  |  |  |  |  |  |
Cumulative adjustment amount to decrease retained earnings |  |  |  | 6.0 |  |  | (492.0)
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings |  |  |  |  |  |  |
New Accounting Pronouncements and Changes in Accounting Principles |  |  |  |  |  |  |
Cumulative adjustment amount to decrease retained earnings |  |  | $ 6.0 | $ 6.0 |  |  | $ (493.0)
Trade Names | Minimum |  |  |  |  |  |  |
Goodwill and Intangible Assets |  |  |  |  |  |  |
Useful life of finite-lived intangible assets | 1 year |  |  |  |  |  |
Trade Names | Maximum |  |  |  |  |  |  |
Goodwill and Intangible Assets |  |  |  |  |  |  |
Useful life of finite-lived intangible assets | 3 years |  |  |  |  |  |
Customer-related and contract-based | Minimum |  |  |  |  |  |  |
Goodwill and Intangible Assets |  |  |  |  |  |  |
Useful life of finite-lived intangible assets | 7 years |  |  |  |  |  |
Customer-related and contract-based | Maximum |  |  |  |  |  |  |
Goodwill and Intangible Assets |  |  |  |  |  |  |
Useful life of finite-lived intangible assets | 20 years |  |  |  |  |  |
Technology and other | Minimum |  |  |  |  |  |  |
Goodwill and Intangible Assets |  |  |  |  |  |  |
Useful life of finite-lived intangible assets | 5 years |  |  |  |  |  |
Technology and other | Maximum |  |  |  |  |  |  |
Goodwill and Intangible Assets |  |  |  |  |  |  |
Useful life of finite-lived intangible assets | 7 years |  |  |  |  |  |
Software | Minimum |  |  |  |  |  |  |
Fixed Assets |  |  |  |  |  |  |
Fixed assets, original life, weighted-average | 4 years |  |  |  |  |  |
Software | Maximum |  |  |  |  |  |  |
Fixed Assets |  |  |  |  |  |  |
Fixed assets, original life, weighted-average | 7 years |  |  |  |  |  |
Leasehold improvements |  |  |  |  |  |  |
Fixed Assets |  |  |  |  |  |  |
Fixed assets, original life, weighted-average | 10 years |  |  |  |  |  |
Furniture, fixtures and equipment | Minimum |  |  |  |  |  |  |
Fixed Assets |  |  |  |  |  |  |
Fixed assets, original life, weighted-average | 4 years |  |  |  |  |  |
Furniture, fixtures and equipment | Maximum |  |  |  |  |  |  |
Fixed Assets |  |  |  |  |  |  |
Fixed assets, original life, weighted-average | 10 years |  |  |  |  |  |
Computer equipment | Minimum |  |  |  |  |  |  |
Fixed Assets |  |  |  |  |  |  |
Fixed assets, original life, weighted-average | 4 years |  |  |  |  |  |
Computer equipment | Maximum |  |  |  |  |  |  |
Fixed Assets |  |  |  |  |  |  |
Fixed assets, original life, weighted-average | 6 years |  |  |  |  |  |
Buildings |  |  |  |  |  |  |
Fixed Assets |  |  |  |  |  |  |
Fixed assets, original life, weighted-average | 35 years |  |  |  |  |  |
Automobiles |  |  |  |  |  |  |
Fixed Assets |  |  |  |  |  |  |
Fixed assets, original life, weighted-average | 6 years |  |  |  |  |  |

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