Income Taxes - Components of Aon's deferred tax assets and liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Employee benefit plans	$ 424	$ 661
Net operating/capital loss and tax credit carryforwards	362	398
Other accrued expenses	65	102
Investment basis differences	35	48
Deferred revenue	20	57
Tradename Liability	12	0
Lease and Service Guarantees	6	0
Brokerage fee arrangements	4	66
Accrued Interest	1	166
Other	48	60
Total	977	1,558
Valuation allowance on deferred tax assets	(136)	(130)
Total	841	1,428
Deferred tax liabilities:
Intangibles and property, plant and equipment	(436)	(978)
Unremitted earnings	(39)	(29)
Deferred costs	(32)	(20)
Unrealized foreign exchange gains	(22)	(26)
Other accrued expenses	(12)	(101)
Other	(38)	(50)
Total	(579)	(1,204)
Net deferred tax asset	$ 262	$ 224