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Summary of Significant Accounting Principles and Practices (Details) $ / shares in Units, £ in Millions, $ in Millions | 3 Months Ended | 12 Months Ended |  |  |  |  |  |
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Dec. 31, 2018 USD ($) $ / shares | Sep. 30, 2018 USD ($) $ / shares | Jun. 30, 2018 USD ($) $ / shares | Mar. 31, 2018 USD ($) $ / shares | Dec. 31, 2017 USD ($) $ / shares | Sep. 30, 2017 USD ($) $ / shares | Jun. 30, 2017 USD ($) $ / shares | Mar. 31, 2017 USD ($) $ / shares | Dec. 31, 2018 USD ($) $ / shares | Dec. 31, 2017 USD ($) $ / shares | Dec. 31, 2016 USD ($) $ / shares | Jan. 01, 2019 USD ($) | Dec. 31, 2018 GBP (£) | Dec. 31, 2018 USD ($) | Jan. 01, 2018 USD ($) | Dec. 31, 2017 GBP (£) | Dec. 31, 2017 USD ($) | Jan. 01, 2017 USD ($) | Dec. 31, 2015 USD ($)
Cash and Cash Equivalents |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash and cash equivalents and short-term investments |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 828.0 |  |  | $ 1,285.0 |  |
Restricted cash and investments, current |  |  |  |  |  |  |  |  |  |  |  |  |  | 91.0 |  |  | 96.0 |  |
Operating funds in U.K. |  |  |  |  |  |  |  |  |  |  |  |  | £ 42.7 | 53.9 |  | £ 42.7 | 57.1 |  |
Fiduciary Assets and Liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premium trust balances |  |  |  |  |  |  |  |  |  |  |  |  |  | 3,900.0 |  |  | 3,700.0 |  |
Allowance for Doubtful Accounts |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Allowance for doubtful accounts |  |  |  |  |  |  |  |  |  |  | $ 56.0 |  |  | 62.0 |  |  | 59.0 |  | $ 58.0
New Accounting Pronouncements and Changes in Accounting Principles [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred tax assets, net |  |  |  |  |  |  |  |  |  |  |  |  |  | 561.0 | $ 261.0 |  | 389.0 |  |
Deferred tax liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  | 181.0 | 169.0 |  | 127.0 |  |
Other non-current assets |  |  |  |  |  |  |  |  |  |  |  |  |  | (448.0) | (452.0) |  | (307.0) |  |
Adoption of new accounting guidance |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 492.0 |  |  | $ 49.0 |
Description of payment terms |  |  |  |  |  |  |  |  | Assets recognized as costs to fulfill a contract, which includes internal costs related to pre-placement broking activities, as well as other costs, are amortized on a systematic basis that is consistent with the transfer of the services to which the asset relates, which is generally less than one year. |  |  |  |  |  |  |  |  |  |
Net income from continuing operations | $ 284.0 | $ 155.0 | $ 57.0 | $ 604.0 | $ 17.0 | $ 196.0 | $ (43.0) | $ 265.0 | $ 1,100.0 | $ 435.0 | $ 1,253.0 |  |  |  |  |  |  |  |
Basic net income per share attributable to Aon shareholders, continuing operations (in dollars per share) | $ / shares | $ 1.14 | $ 0.61 | $ 0.19 | $ 2.37 | $ 0.04 | $ 0.74 | $ (0.20) | $ 0.95 | $ 4.32 | $ 1.54 | $ 4.55 |  |  |  |  |  |  |  |
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 2016-16 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements and Changes in Accounting Principles [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred tax assets, net |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 23.0 |  |
Deferred tax liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 12.0 |  |
Other non-current assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 26.0 |  |
Retained Earnings |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements and Changes in Accounting Principles [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Adoption of new accounting guidance |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 493.0 |  |  | $ 49.0 |
Retained Earnings | Accounting Standards Update 2016-16 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements and Changes in Accounting Principles [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Adoption of new accounting guidance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (15.0) |  |
Retained Earnings | Accounting Standards Update 2016-01 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements and Changes in Accounting Principles [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Adoption of new accounting guidance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.0 |  |
Accumulated Other Comprehensive Loss, Net of Tax |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements and Changes in Accounting Principles [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Adoption of new accounting guidance |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (1.0) |  |  |  |
Accumulated Other Comprehensive Loss, Net of Tax | Accounting Standards Update 2016-01 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements and Changes in Accounting Principles [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Adoption of new accounting guidance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (1.0) |  |
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 | 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 |  |  |  |  |  |  |  |  |  |
Scenario, Forecast | Accounting Standards Update 2016-02 | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements and Changes in Accounting Principles [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating lease liability |  |  |  |  |  |  |  |  |  |  |  | $ 1,300.0 |  |  |  |  |  |  |
Lease right-of-use asset |  |  |  |  |  |  |  |  |  |  |  | 1,100.0 |  |  |  |  |  |  |
Scenario, Forecast | Accounting Standards Update 2016-02 | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements and Changes in Accounting Principles [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating lease liability |  |  |  |  |  |  |  |  |  |  |  | 1,500.0 |  |  |  |  |  |  |
Lease right-of-use asset |  |  |  |  |  |  |  |  |  |  |  | $ 1,300.0 |  |  |  |  |  |  |
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements and Changes in Accounting Principles [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred tax assets, net |  |  |  |  |  |  |  |  |  |  |  |  |  | 137.0 | (128.0) |  |  |  |
Deferred tax liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  | (28.0) | 42.0 |  |  |  |
Other non-current assets |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 155.0 | $ (145.0) |  |  |  |
Net income from continuing operations |  |  |  |  |  |  |  |  | $ (78.0) |  |  |  |  |  |  |  |  |  |
Basic net income per share attributable to Aon shareholders, continuing operations (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  | $ 0.32 |  |  |  |  |  |  |  |  |  |

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