Accelerating Aon United Program - Schedule of Changes in the Company’s Liabilities for the Program (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Restructuring Reserve [Roll Forward]
Charges	$ 69	$ 6	$ 320	$ 6
Accelerating Aon United Program
Restructuring Reserve [Roll Forward]
Liability balance as of January 1, 2024			100
Charges	69		320
Cash payments			(183)
Foreign currency translation and other			2
Non-cash charges			(94)
Liability balance as of September 30, 2024	145		145
Total costs incurred from inception to date	455		455
Asset amortization	4		24
Accelerating Aon United Program | Technology and other
Restructuring Reserve [Roll Forward]
Liability balance as of January 1, 2024			14
Charges			87
Cash payments			(63)
Foreign currency translation and other			0
Non-cash charges			(24)
Liability balance as of September 30, 2024	14		14
Total costs incurred from inception to date	101		101
Accelerating Aon United Program | Workforce optimization
Restructuring Reserve [Roll Forward]
Liability balance as of January 1, 2024			86
Charges			173
Cash payments			(120)
Foreign currency translation and other			2
Non-cash charges			(10)
Liability balance as of September 30, 2024	131		131
Total costs incurred from inception to date	276		276
Accelerating Aon United Program | Asset impairments
Restructuring Reserve [Roll Forward]
Liability balance as of January 1, 2024			0
Charges			60
Cash payments			0
Foreign currency translation and other			0
Non-cash charges			(60)
Liability balance as of September 30, 2024	0		0
Total costs incurred from inception to date	$ 78		$ 78