Accelerating Aon United Program - Schedule of Changes in the Company’s Liabilities for the Program (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2025	Mar. 31, 2024
Restructuring Reserve [Roll Forward]
Charges	$ 110	$ 119
Accelerating Aon United Program
Restructuring Reserve [Roll Forward]
Liability balance as of December 31, 2024	114
Charges	110	$ 119
Cash payments	(97)
Foreign currency translation and other	0
Non-cash charges	(19)
Liability balance as of March 31, 2025	108
Total costs incurred from inception to date	634
Asset amortization	2
Accelerating Aon United Program \| Technology and other
Restructuring Reserve [Roll Forward]
Liability balance as of December 31, 2024	17
Charges	56
Cash payments	(45)
Foreign currency translation and other	0
Non-cash charges	(2)
Liability balance as of March 31, 2025	26
Total costs incurred from inception to date	196
Accelerating Aon United Program \| Workforce optimization
Restructuring Reserve [Roll Forward]
Liability balance as of December 31, 2024	97
Charges	51
Cash payments	(52)
Foreign currency translation and other	0
Non-cash charges	(14)
Liability balance as of March 31, 2025	82
Total costs incurred from inception to date	351
Accelerating Aon United Program \| Asset impairments
Restructuring Reserve [Roll Forward]
Liability balance as of December 31, 2024	0
Charges	3
Cash payments	0
Foreign currency translation and other	0
Non-cash charges	(3)
Liability balance as of March 31, 2025	0
Total costs incurred from inception to date	$ 87

Amount of periodic reduction over lease term of carrying amount of right-of-use asset from operating lease.

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