Accelerating Aon United Program - Changes in the Company’s Liabilities for the Program (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Restructuring Reserve [Roll Forward]
Charges	$ 119	$ 0
Accelerating Aon United Program
Restructuring Reserve [Roll Forward]
Liability Balance as of January 1, 2024	100
Charges	119
Cash payments	(37)
Foreign currency translation and other	(1)
Non-cash charges	(48)
Liability balance as of March 31, 2024	133
Liability balance as of March 31, 2024	254
Accelerating Aon United Program | Technology and other
Restructuring Reserve [Roll Forward]
Liability Balance as of January 1, 2024	14
Charges	19
Cash payments	(12)
Foreign currency translation and other	0
Non-cash charges	(7)
Liability balance as of March 31, 2024	14
Liability balance as of March 31, 2024	33
Accelerating Aon United Program | Workforce optimization
Restructuring Reserve [Roll Forward]
Liability Balance as of January 1, 2024	86
Charges	64
Cash payments	(25)
Foreign currency translation and other	(1)
Non-cash charges	(5)
Liability balance as of March 31, 2024	119
Liability balance as of March 31, 2024	167
Accelerating Aon United Program | Asset impairments
Restructuring Reserve [Roll Forward]
Liability Balance as of January 1, 2024	0
Charges	36
Cash payments	0
Foreign currency translation and other	0
Non-cash charges	(36)
Liability balance as of March 31, 2024	0
Liability balance as of March 31, 2024	$ 54