Summary of Significant Accounting Principles and Practices (Details) $ / shares in Units, £ in Millions, $ in Millions	3 Months Ended								12 Months Ended
	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