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

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The aggregate foreign currency transaction gain (loss) (both realized and unrealized) included in determining net income for the reporting period. Includes foreign currency transactions designated as hedges.

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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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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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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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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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Amount, after deferred tax asset, of deferred tax liability attributable to taxable differences, with jurisdictional netting and classified as noncurrent.

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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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Gain (loss) incurred in hedging activities.

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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 carrying amounts of cash and cash equivalent items which 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. For a classified balance sheet represents the current portion only (the noncurrent portion has a separate concept); there is a separate and distinct element for unclassified presentations.

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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) £ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jan. 01, 2017 USD ($)	Dec. 31, 2016 GBP (£)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 GBP (£)	Dec. 31, 2015 USD ($)	Dec. 31, 2013 USD ($)
Cash and Cash Equivalents
Cash and cash equivalents and short-term investments						$ 721.0		$ 740.0
Restricted cash and investments, current						82.0		105.0
Operating funds in U.K.					£ 43.3	53.2	£ 43.3	64.6
Cash and cash equivalents, restricted						29.0		40.0
Fiduciary Assets and Liabilities
Premium trust balances						3,800.0		3,400.0
Allowance for Doubtful Accounts
Allowance for doubtful accounts			$ 74.0			58.0		58.0	$ 90.0
Foreign Currency
Effect of foreign exchange gains (losses) on the consolidated statements of income	$ 9.0	$ 11.0	1.0
Loss on hedging activity	$ 7.0	$ 19.0	$ 19.0
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Deferred tax assets, net						322.0		300.0
Deferred tax liabilities, net, current						$ 101.0		37.0
Technology equipment
Fixed Assets
Fixed assets, original life, weighted-average	7 years
Buildings
Fixed Assets
Fixed assets, original life, weighted-average	35 years
Automobiles
Fixed Assets
Fixed assets, original life, weighted-average	6 years
Minimum
Revenue Recognition
Term of outsourcing contracts for benefit services	3 years
Goodwill and Intangible Assets
Useful life of finite-lived intangible assets	1 year
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
Revenue Recognition
Term of outsourcing contracts for benefit services	5 years
Goodwill and Intangible Assets
Useful life of finite-lived intangible assets	16 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
Weighted average
Goodwill and Intangible Assets
Useful life of finite-lived intangible assets	11 years
Other Current Assets \| Accounting Standard Update 2015-03
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Debt issuance costs, net								4.0
Other Noncurrent Assets \| Accounting Standard Update 2015-03
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Debt issuance costs, net								33.0
New Accounting Pronouncement, Early Adoption, Effect \| Accounting Standards Update 2015-07
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Deferred tax assets, net								(93.0)
Deferred tax liabilities, net, current								$ (139.0)
Retained Earnings \| Subsequent Event \| New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Adjustments to retained earnings for deferred tax asset changes due to share-based compensation changes				$ 49.0