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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 |  |  |  |  |

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