Accounting Principles and Practices - Schedule of Impact of Topic 606 on Statement of Income (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue
Total revenue	$ 2,349	$ 2,340	$ 8,000	$ 7,089
Expenses
Compensation and benefits	1,392	1,428	4,502	4,363
Other general expenses	336	317	1,189	956
Income tax expense (benefit)	39	4	9	(139)
Net income from continuing operations	$ (155)	$ (196)	$ (816)	$ (418)
Basic net income (loss) per share attributable to Aon shareholders, continuing operations (in dollars per share)	$ 0.61	$ 0.74	$ 3.18	$ 1.49
Calculated under Revenue Guidance in Effect before Topic 606
Revenue
Total revenue	$ 2,491		$ 7,732
Expenses
Compensation and benefits	1,428		4,460
Other general expenses	337		1,192
Income tax expense (benefit)	60		(45)
Accounting Standards Update 2014-09 | Adjustments
Revenue
Total revenue	142		(268)
Expenses
Compensation and benefits	36		(42)
Other general expenses	1		3
Income tax expense (benefit)	21		(54)
Net income from continuing operations	$ 84		$ (175)
Basic net income (loss) per share attributable to Aon shareholders, continuing operations (in dollars per share)	$ 0.34		$ 0.71