Income Taxes - Components of Aon's deferred tax assets and liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Deferred tax assets:
Net operating loss, capital loss, interest, and tax credit carryforwards	$ 581	$ 653
Lease liabilities	207	248
Employee benefit plans	160	312
Other accrued expenses	132	103
Accrued interest	97	0
Federal and state benefit of interest from uncertain tax positions	45	37
Deferred revenue	36	36
Investment basis differences	25	28
Lease and service guarantees	1	2
Other	25	17
Total	1,309	1,436
Valuation allowance on deferred tax assets	(230)	(205)
Total	1,079	1,231
Deferred tax liabilities:
Intangibles and property, plant and equipment	(243)	(291)
Lease right-of-use asset	(173)	(211)
Deferred costs	(159)	(141)
Unremitted earnings	(58)	(37)
Other accrued expenses	(27)	(22)
Unrealized foreign exchange gains	(22)	(26)
Other	(32)	(41)
Total	(714)	(769)
Net deferred tax asset	$ 365	462
Federal and state benefit of interest from uncertain tax positions		$ 37