Cash and Cash Equivalents and Short-Term Investments

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Represents cash that is restricted for minimum operating fund requirements.

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Amount of allowance for credit loss on accounts receivable, classified as current.

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Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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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.

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Amount of stockholders' equity (deficit), net of receivables from officers, directors, owners, and affiliates of the entity, attributable to both the parent and noncontrolling interests. Amount excludes temporary equity. Alternate caption for the concept is permanent equity.

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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