Accelerating Aon United Program (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Restructuring Reserve [Roll Forward]
Charges	$ 135	$ 0	$ 0
Accelerating Aon United Program
Restructuring Reserve [Roll Forward]
Liability Balance as of January 1, 2023	0
Charges	135
Cash payments	(13)
Foreign currency translation and other	1
Non-cash charges	(23)
Liability balance as of December 31, 2023	100	0
Total costs incurred from inception to date	135
Accelerating Aon United Program | Technology and other
Restructuring Reserve [Roll Forward]
Liability Balance as of January 1, 2023	0
Charges	14
Cash payments	0
Foreign currency translation and other	0
Non-cash charges	0
Liability balance as of December 31, 2023	14	0
Total costs incurred from inception to date	14
Accelerating Aon United Program | Workforce optimization
Restructuring Reserve [Roll Forward]
Liability Balance as of January 1, 2023	0
Charges	103
Cash payments	(13)
Foreign currency translation and other	1
Non-cash charges	(5)
Liability balance as of December 31, 2023	86	0
Total costs incurred from inception to date	103
Accelerating Aon United Program | Asset impairments
Restructuring Reserve [Roll Forward]
Liability Balance as of January 1, 2023	0
Charges	18
Cash payments	0
Foreign currency translation and other	0
Non-cash charges	(18)
Liability balance as of December 31, 2023	0	$ 0
Total costs incurred from inception to date	$ 18