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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 |  |  |  |  |  |

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