Accelerating Aon United Program - Schedule of Changes in the Company’s Liabilities for the Program (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Restructuring Reserve [Roll Forward]
Charges	$ 32	$ 69	$ 236	$ 320
Accelerating Aon United Program
Restructuring Reserve [Roll Forward]
Liability balance as of December 31, 2024			114
Charges	32	$ 69	236	$ 320
Cash payments			(234)
Foreign currency translation and other			5
Non-cash charges			(31)
Liability balance as of September 30, 2025	90		90
Total costs incurred from inception to date	760		760
Accelerating Aon United Program | Technology and other
Restructuring Reserve [Roll Forward]
Liability balance as of December 31, 2024			17
Charges			138
Cash payments			(124)
Foreign currency translation and other			0
Non-cash charges			(7)
Liability balance as of September 30, 2025	24		24
Total costs incurred from inception to date	278		278
Accelerating Aon United Program | Workforce optimization
Restructuring Reserve [Roll Forward]
Liability balance as of December 31, 2024			97
Charges			91
Cash payments			(110)
Foreign currency translation and other			5
Non-cash charges			(17)
Liability balance as of September 30, 2025	66		66
Total costs incurred from inception to date	391		391
Accelerating Aon United Program | Asset impairments
Restructuring Reserve [Roll Forward]
Liability balance as of December 31, 2024			0
Charges			7
Cash payments			0
Foreign currency translation and other			0
Non-cash charges			(7)
Liability balance as of September 30, 2025	0		0
Total costs incurred from inception to date	$ 91		$ 91