Accounting Principles and Practices - Schedule of Impact of Topic 606 on Statement of Income (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue
Total revenue	$ 2,561	$ 2,368	$ 5,651	$ 4,749
Expenses
Compensation and benefits	1,494	1,466	3,110	2,935
Other general expenses	535	331	853	639
Other income (expense)	(3)	4	(18)	2
Income tax benefit	(144)	(143)	(30)	(143)
Net income from continuing operations	$ 57	$ (43)	$ 661	$ 222
Basic net income (loss) per share attributable to Aon shareholders, continuing operations (in dollars per share)	$ 0.19	$ (0.20)	$ 2.57	$ 0.75
Calculated under Revenue Guidance in Effect before Topic 606
Revenue
Total revenue	$ 2,564		$ 5,241
Expenses
Compensation and benefits	1,481		3,032
Other general expenses	537		855
Other income (expense)	(4)		(18)
Income tax benefit	(137)		(105)
Accounting Standards Update 2014-09 | Adjustments
Revenue
Total revenue	3		(410)
Expenses
Compensation and benefits	(13)		(78)
Other general expenses	2		2
Other income (expense)	(1)		0
Income tax benefit	7		(75)
Net income from continuing operations	$ (6)		$ 259
Basic net income (loss) per share attributable to Aon shareholders, continuing operations (in dollars per share)	$ (0.02)		$ 1.05