Income Taxes - Components of Aon's deferred tax assets and liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Deferred tax assets:
Net operating loss, capital loss, interest, and tax credit carryforwards	$ 952	$ 581
Lease liabilities	178	207
Employee benefit plans	297	160
Other accrued expenses	100	132
Accrued interest	0	97
Federal and state benefit of interest from uncertain tax positions	57	45
Deferred revenue	26	36
Investment basis differences	30	25
Lease and service guarantees	1	1
Other	38	25
Total	1,679	1,309
Valuation allowance on deferred tax assets	(275)	(230)
Total	1,404	1,079
Deferred tax liabilities:
Intangibles and property, plant and equipment	(258)	(243)
Lease right-of-use asset	(151)	(173)
Deferred costs	(147)	(159)
Unremitted earnings	(38)	(58)
Other accrued expenses	(20)	(27)
Unrealized foreign exchange gains	(23)	(22)
Other	(42)	(32)
Total	(679)	(714)
Net deferred tax asset	$ 725	$ 365