Income Taxes - Components of Aon's Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Deferred tax assets:
Net operating loss, capital loss, interest, and tax credit carryforwards	$ 1,682	$ 1,576
Employee benefit plans	325	359
Lease liabilities	157	171
Other accrued expenses	166	176
Federal and state benefit of interest from uncertain tax positions	127	99
Accrued interest	16	102
Deferred revenue	34	31
Investment basis differences	5	(42)
Other	58	56
Total	2,570	2,528
Valuation allowance on deferred tax assets	(306)	(202)
Total	2,264	2,326
Deferred tax liabilities:
Intangibles and property, plant and equipment	(1,367)	(1,512)
Deferred costs	(184)	(160)
Lease right-of-use asset	(125)	(139)
Unremitted earnings	(55)	(66)
Other accrued expenses	(23)	(21)
Unrealized foreign exchange gains	(41)	(36)
Other	(61)	(57)
Total	(1,856)	(1,991)
Net deferred tax asset	$ 408	$ 335