Income Taxes - Components of Aon's Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Deferred tax assets:
Net operating loss, capital loss, interest, and tax credit carryforwards	$ 1,576	$ 1,049
Employee benefit plans	359	337
Lease liabilities	171	164
Other accrued expenses	176	155
Federal and state benefit of interest from uncertain tax positions	99	75
Accrued interest	102	52
Deferred revenue	31	25
Other	56	40
Total	2,570	1,897
Valuation allowance on deferred tax assets	(202)	(197)
Total	2,368	1,700
Deferred tax liabilities:
Intangibles and property, plant and equipment	(1,512)	(254)
Deferred costs	(160)	(149)
Lease right-of-use asset	(139)	(135)
Unremitted earnings	(66)	(44)
Other accrued expenses	(21)	(20)
Unrealized foreign exchange gains	(36)	(18)
Investment basis differences	42
Investment basis differences		49
Other	(57)	(49)
Total	(2,033)	(620)
Net deferred tax asset	$ 335	$ 1,080