Income Taxes - Components of Aon's deferred tax assets and liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss, capital loss, interest, and tax credit carryforwards	$ 563	$ 362
Employee benefit plans	351	424
Other accrued expenses	98	65
Investment basis differences	28	35
Deferred revenue	29	20
Tradename liability	0	12
Lease and service guarantees	5	6
Brokerage fee arrangements	0	4
Other	46	49
Total	1,120	977
Valuation allowance on deferred tax assets	(171)	(136)
Total	949	841
Deferred tax liabilities:
Intangibles and property, plant and equipment	(310)	(436)
Deferred costs	(143)	(32)
Unremitted earnings	(30)	(39)
Unrealized foreign exchange gains	(26)	(22)
Other accrued expenses	(36)	(12)
Other	(24)	(38)
Total	(569)	(579)
Net deferred tax asset	$ 380	$ 262