Accelerating Aon United Program - Schedule of Changes in the Company’s Liabilities for the Program (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Restructuring Reserve [Roll Forward]
Charges	$ 94	$ 132	$ 204	$ 251
Accelerating Aon United Program
Restructuring Reserve [Roll Forward]
Liability balance as of December 31, 2024			114
Charges	94	$ 132	204	$ 251
Cash payments			(166)
Foreign currency translation and other			5
Non-cash charges			(23)
Liability balance as of June 30, 2025	134		134
Total costs incurred from inception to date	728		728
Accelerating Aon United Program | Technology and other
Restructuring Reserve [Roll Forward]
Liability balance as of December 31, 2024			17
Charges			100
Cash payments			(84)
Foreign currency translation and other			0
Non-cash charges			(2)
Liability balance as of June 30, 2025	31		31
Total costs incurred from inception to date	240		240
Accelerating Aon United Program | Workforce optimization
Restructuring Reserve [Roll Forward]
Liability balance as of December 31, 2024			97
Charges			100
Cash payments			(82)
Foreign currency translation and other			5
Non-cash charges			(17)
Liability balance as of June 30, 2025	103		103
Total costs incurred from inception to date	400		400
Accelerating Aon United Program | Asset impairments
Restructuring Reserve [Roll Forward]
Liability balance as of December 31, 2024			0
Charges			4
Cash payments			0
Foreign currency translation and other			0
Non-cash charges			(4)
Liability balance as of June 30, 2025	0		0
Total costs incurred from inception to date	$ 88		$ 88