Accounting Principles and Practices - Schedule of Impact of Topic 606 on Statement of Income (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue
Total revenue	$ 3,090	$ 2,381
Expenses
Compensation and benefits	1,616	1,469
Other income (expense)	(15)	(2)
Income taxes	114	0
Net income from continuing operations	$ 604	$ 265
Basic net income (loss) per share attributable to Aon shareholders, continuing operations (in dollars per share)	$ 2.37	$ 0.95
Calculated under Revenue Guidance in Effect before Topic 606
Revenue
Total revenue	$ 2,677
Expenses
Compensation and benefits	1,551
Other income (expense)	(14)
Income taxes	32
Accounting Standards Update 2014-09 | Adjustments
Revenue
Total revenue	(413)
Expenses
Compensation and benefits	(65)
Other income (expense)	1
Income taxes	(82)
Net income from continuing operations	$ 265
Basic net income (loss) per share attributable to Aon shareholders, continuing operations (in dollars per share)	$ 1.06