Shareholders' Equity - Components of Accumulated other comprehensive loss, net of related tax (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 3,449	$ 4,219	$ 4,648
Other comprehensive income (loss) before reclassifications	4	(282)	(576)
Tax benefit (expense)	62	58	43
Other comprehensive income (loss) before reclassifications, net	66	(224)	(533)
Amounts reclassified from accumulated other comprehensive income (loss)	140	123	157
Tax benefit (expense)	(34)	(23)	(36)
Amounts reclassified from accumulated other comprehensive income (loss), net	106	100	121
Total other comprehensive income (loss) attributable to Aon shareholders	172	(124)	(412)
Ending Balance	3,583	3,449	4,219
Cumulative Effect, Period of Adoption, Adjustment
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(6)		492
Ending Balance		(6)
Cumulative Effect, Period of Adoption, Adjusted Balance
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	3,443		5,140
Ending Balance		3,443
Change in Fair Value of Investments
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(12)	(15)	(25)
Other comprehensive income (loss) before reclassifications	1	(10)	(15)
Tax benefit (expense)	0	1	18
Other comprehensive income (loss) before reclassifications, net	1	(9)	3
Amounts reclassified from accumulated other comprehensive income (loss)	15	14	11
Tax benefit (expense)	(3)	(2)	(3)
Amounts reclassified from accumulated other comprehensive income (loss), net	12	12	8
Total other comprehensive income (loss) attributable to Aon shareholders	13	3	11
Ending Balance	1	(12)	(15)
Change in Fair Value of Investments | Cumulative Effect, Period of Adoption, Adjustment
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(1)
Change in Fair Value of Investments | Cumulative Effect, Period of Adoption, Adjusted Balance
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(26)
Foreign Currency Translation Adjustments
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(1,305)	(1,319)	(879)
Other comprehensive income (loss) before reclassifications	258	15	(437)
Tax benefit (expense)	2	(1)	(3)
Other comprehensive income (loss) before reclassifications, net	260	14	(440)
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	0
Tax benefit (expense)	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss), net	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	260	14	(440)
Ending Balance	(1,045)	(1,305)	(1,319)
Foreign Currency Translation Adjustments | Cumulative Effect, Period of Adoption, Adjustment
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			0
Foreign Currency Translation Adjustments | Cumulative Effect, Period of Adoption, Adjusted Balance
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(879)
Post-retirement Benefit Obligations
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(2,716)	(2,575)	(2,592)
Other comprehensive income (loss) before reclassifications	(255)	(287)	(124)
Tax benefit (expense)	60	58	28
Other comprehensive income (loss) before reclassifications, net	(195)	(229)	(96)
Amounts reclassified from accumulated other comprehensive income (loss)	125	109	146
Tax benefit (expense)	(31)	(21)	(33)
Amounts reclassified from accumulated other comprehensive income (loss), net	94	88	113
Total other comprehensive income (loss) attributable to Aon shareholders	(101)	(141)	17
Ending Balance	(2,817)	(2,716)	(2,575)
Post-retirement Benefit Obligations | Cumulative Effect, Period of Adoption, Adjustment
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			0
Post-retirement Benefit Obligations | Cumulative Effect, Period of Adoption, Adjusted Balance
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(2,592)
AOCI attributable to parent
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(4,033)	(3,909)	(3,496)
Ending Balance	(3,861)	(4,033)	(3,909)
AOCI attributable to parent | Cumulative Effect, Period of Adoption, Adjustment
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(1)
AOCI attributable to parent | Cumulative Effect, Period of Adoption, Adjusted Balance
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ (4,033)		$ (3,497)
Ending Balance		$ (4,033)