Accelerating Aon United Program - Schedule of Changes in the Company’s Liabilities for the Program (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Restructuring Reserve [Roll Forward]
Charges	$ 92	$ 110
Accelerating Aon United Program
Restructuring Reserve [Roll Forward]
Liability balance as of December 31, 2025	147
Charges	92	$ 110
Cash payments	(85)
Foreign currency translation and other	(1)
Non-cash charges	7
Liability balance as of March 31, 2026	160
Total costs incurred from inception to date	981
Accelerating Aon United Program | Technology and other
Restructuring Reserve [Roll Forward]
Liability balance as of December 31, 2025	39
Charges	62
Cash payments	(51)
Foreign currency translation and other	0
Non-cash charges	0
Liability balance as of March 31, 2026	50
Total costs incurred from inception to date	397
Accelerating Aon United Program | Workforce optimization
Restructuring Reserve [Roll Forward]
Liability balance as of December 31, 2025	108
Charges	29
Cash payments	(34)
Foreign currency translation and other	(1)
Non-cash charges	8
Liability balance as of March 31, 2026	110
Total costs incurred from inception to date	484
Accelerating Aon United Program | Asset impairments
Restructuring Reserve [Roll Forward]
Liability balance as of December 31, 2025	0
Charges	1
Cash payments	0
Foreign currency translation and other	0
Non-cash charges	(1)
Liability balance as of March 31, 2026	0
Total costs incurred from inception to date	$ 100