Income Taxes - Components of Aon's deferred tax assets and liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating loss, capital loss, interest, and tax credit carryforwards	$ 653	$ 440
Employee benefit plans	312	373
Lease liabilities	248	247
Other accrued expenses	103	68
Deferred revenue	36	24
Investment basis differences	28	28
Lease and service guarantees	2	3
Accrued interest	0	116
Other	54	57
Total	1,436	1,356
Valuation allowance on deferred tax assets	(205)	(200)
Total	1,231	1,156
Deferred tax liabilities:
Intangibles and property, plant and equipment	(291)	(251)
Lease right-of-use asset	(211)	(219)
Deferred costs	(141)	(128)
Unremitted earnings	(37)	(28)
Unrealized foreign exchange gains	(26)	(26)
Other accrued expenses	(22)	(25)
Other	(41)	(33)
Total	(769)	(710)
Net deferred tax asset	$ 462	$ 446