Accelerating Aon United Program - Restructuring Liabilities for the Program (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Restructuring Reserve [Roll Forward]
Charges		$ 389	$ 135	$ 0
Operating lease ROU asset expense		186	141
Accelerating Aon United Program
Restructuring Reserve [Roll Forward]
Liability balance as of January 1, 2024		100
Charges		389	135
Cash payments		(268)
Foreign currency translation and other		(3)
Non-cash charges		(104)
Liability balance as of December 31, 2024	$ 114	114	100
Total costs incurred from inception to date	524	524
Operating lease ROU asset expense	27
Accelerating Aon United Program | Technology and other
Restructuring Reserve [Roll Forward]
Liability balance as of January 1, 2024		14
Charges		126
Cash payments		(96)
Foreign currency translation and other		0
Non-cash charges		(27)
Liability balance as of December 31, 2024	17	17	14
Total costs incurred from inception to date	140	140
Accelerating Aon United Program | Workforce optimization
Restructuring Reserve [Roll Forward]
Liability balance as of January 1, 2024		86
Charges		197
Cash payments		(172)
Foreign currency translation and other		(3)
Non-cash charges		(11)
Liability balance as of December 31, 2024	97	97	86
Total costs incurred from inception to date	300	300
Accelerating Aon United Program | Asset impairments
Restructuring Reserve [Roll Forward]
Liability balance as of January 1, 2024		0
Charges		66
Cash payments		0
Foreign currency translation and other		0
Non-cash charges		(66)
Liability balance as of December 31, 2024	0	0	$ 0
Total costs incurred from inception to date	$ 84	$ 84