Income Taxes - Components of Aon's deferred tax assets and liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets:
Net operating loss, capital loss, interest, and tax credit carryforwards	$ 1,049	$ 952
Employee benefit plans	337	297
Lease liabilities	164	178
Other accrued expenses	155	100
Federal and state benefit of interest from uncertain tax positions	75	57
Accrued interest	52	0
Deferred revenue	25	26
Investment basis differences	49	30
Other	40	39
Total	1,946	1,679
Valuation allowance on deferred tax assets	(197)	(275)
Total	1,749	1,404
Deferred tax liabilities:
Intangibles and property, plant and equipment	(254)	(258)
Deferred costs	(149)	(147)
Lease right-of-use asset	(135)	(151)
Unremitted earnings	(44)	(38)
Other accrued expenses	(20)	(20)
Unrealized foreign exchange gains	(18)	(23)
Other	(49)	(42)
Total	(669)	(679)
Net deferred tax asset	$ 1,080	725
Lease and service guarantees		$ 1