Debt - Schedule of Commercial Paper (Details) - Commercial paper - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Debt Instrument [Line Items]
Weighted average commercial paper outstanding	$ 17	$ 456
Weighted average interest rate of commercial paper outstanding	0.19%	0.96%

Reflects the calculation as of the balance sheet date of the average interest rate weighted by the average outstanding amount of short-term debt outstanding by type or by instrument at that time.

No definition available.

Name:	aon_ShortTermDebtAverageOutstandingAmountWeightedAverageInterestRate
Namespace Prefix:	aon_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration