Cash and Cash Equivalents and Short-Term Investments

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Effective Income Tax Reconciliation, Share-based Compensation, Excess Tax Benefit Amount

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Represents premium trust balances for premiums collected from insureds but not yet remitted to insurance companies.

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Represents cash that is restricted for minimum operating fund requirements.

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A valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.4)
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
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Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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No definition available.

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Amount, after allocation of valuation allowances and deferred tax liability, of deferred tax asset attributable to deductible differences and carryforwards, with jurisdictional netting.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319

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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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The tax effect on Income or Loss from extraordinary items per each share of common stock outstanding during the reporting period.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=96947427&loc=d3e1278-109256

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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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Summary of Significant Accounting Principles and Practices (Details) $ / shares in Units, £ in Millions, $ in Millions		12 Months Ended
	May 01, 2017	Dec. 31, 2017 USD ($) $ / shares	Jan. 01, 2018 USD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 GBP (£)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Cash and Cash Equivalents
Cash and cash equivalents and short-term investments					$ 1,285.0		$ 716.0
Restricted cash and investments, current					96.0		82.0
Operating funds in U.K.				£ 42.7	57.1	£ 43.3	53.2
Fiduciary Assets and Liabilities
Premium trust balances					3,700.0		3,300.0
Allowance for Doubtful Accounts
Allowance for doubtful accounts					59.0		56.0	$ 58.0	$ 72.0
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Adoption of new accounting guidance							49.0
Deferred tax assets, net					$ 389.0		325.0
Excess tax benefit amount		$ 54.0
Buildings
Fixed Assets
Fixed assets, original life, weighted-average		35 years
Automobiles
Fixed Assets
Fixed assets, original life, weighted-average		6 years
Minimum \| Software
Fixed Assets
Fixed assets, original life, weighted-average		4 years
Minimum \| Furniture, fixtures and equipment
Fixed Assets
Fixed assets, original life, weighted-average		4 years
Minimum \| Computer equipment
Fixed Assets
Fixed assets, original life, weighted-average		4 years
Maximum \| Software
Fixed Assets
Fixed assets, original life, weighted-average		7 years
Maximum \| Leasehold improvements
Fixed Assets
Fixed assets, original life, weighted-average		10 years
Maximum \| Furniture, fixtures and equipment
Fixed Assets
Fixed assets, original life, weighted-average		10 years
Maximum \| Computer equipment
Fixed Assets
Fixed assets, original life, weighted-average		6 years
Accounting Standards Update 2016-09, Excess Tax Benefit Component [Member]
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Deferred tax assets, net							49.0
Income (loss) from extraordinary items, tax effect (in dollars per share) \| $ / shares		$ 0.21
Retained Earnings
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Adoption of new accounting guidance							49.0
Retained Earnings \| Accounting Standards Update 2016-09, Excess Tax Benefit Component [Member]
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Adoption of new accounting guidance							$ 49.0
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
Tradenames
Goodwill and Intangible Assets
Useful life of finite-lived intangible assets	3 years
Tradenames \| Minimum
Goodwill and Intangible Assets
Useful life of finite-lived intangible assets		1 year
Tradenames \| Maximum
Goodwill and Intangible Assets
Useful life of finite-lived intangible assets		3 years
Technology \| Minimum
Goodwill and Intangible Assets
Useful life of finite-lived intangible assets		5 years
Technology \| Maximum
Goodwill and Intangible Assets
Useful life of finite-lived intangible assets		7 years
Subsequent Event \| Accounting Standards Update 2016-16
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Adoption of new accounting guidance			$ 15.0
Subsequent Event \| Accounting Standards Update 2014-09 \| Minimum
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Adoption of new accounting guidance			400.0
Subsequent Event \| Accounting Standards Update 2014-09 \| Maximum
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Adoption of new accounting guidance			$ 600.0