Accelerating Aon United Program - Narrative (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2025	Sep. 30, 2024
Restructuring Cost and Reserve [Line Items]
Charges	$ 32	$ 69		$ 236	$ 320
Accelerating Aon United Program
Restructuring Cost and Reserve [Line Items]
Restructuring plan, period			3 years
Expected cost	1,000			1,000
Expected cost, cash charges	900			900
Expected cost, non-cash charges	100			100
Charges	32	$ 69		236	$ 320
Accelerating Aon United Program | Risk Capital
Restructuring Cost and Reserve [Line Items]
Expected cost	230			230
Accelerating Aon United Program | Human Capital
Restructuring Cost and Reserve [Line Items]
Expected cost	$ 60			$ 60