Accelerating Aon United Program - Program Costs Incurred (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Restructuring Cost and Reserve [Line Items]
Accelerating Aon United Program expenses	$ 389	$ 135	$ 0
Accelerating Aon United Program
Restructuring Cost and Reserve [Line Items]
Accelerating Aon United Program expenses	389	135
Accelerating Aon United Program | Corporate
Restructuring Cost and Reserve [Line Items]
Accelerating Aon United Program expenses	248	55
Risk Capital | Accelerating Aon United Program | Operating Segments
Restructuring Cost and Reserve [Line Items]
Accelerating Aon United Program expenses	114	57
Human Capital | Accelerating Aon United Program | Operating Segments
Restructuring Cost and Reserve [Line Items]
Accelerating Aon United Program expenses	$ 27	$ 23