Accelerating Aon United Program - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Sep. 30, 2023	Mar. 31, 2023
Restructuring Cost and Reserve [Line Items]
Charges	$ 119		$ 0
Accelerating Aon United Program
Restructuring Cost and Reserve [Line Items]
Restructuring plan, period		3 years
Expected cost	1,000
Expected cost, cash charges	900
Expected cost, non-cash charges	100
Charges	$ 119