Derivatives and Hedging - Foreign Exchange Risk Management Narrative (Details)	6 Months Ended
Jun. 30, 2018
Cash Flow Hedging
Derivative [Line Items]
Foreign currency exposures, maximum average hedging period (less than)	2 years
Not Designated as Hedging Instrument
Derivative [Line Items]
Foreign currency exposures, maximum hedging period (up to)	1 year

Maximum length of time over which the entity is hedging its exposure to the variability in future exchange rates using instruments not designated as accounting hedges.

No definition available.

Name:	aon_MaximumLengthOfTimeHedgedInForeignCurrencyUndesignatedHedge
Namespace Prefix:	aon_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration