Shareholders' Equity (Deficit) - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	$ (429)	$ 1,158
Other comprehensive income (loss) before reclassifications, net	53	4
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income	37	32
Tax expense	(11)	(8)
Amounts reclassified from accumulated other comprehensive income, net	26	24
Total other comprehensive income attributable to Aon shareholders	79	28
Ending Balance	58	1,291
Change in Fair Value of Financial Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(11)	2
Other comprehensive income (loss) before reclassifications, net	0	0
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income	5	1
Tax expense	(2)	0
Amounts reclassified from accumulated other comprehensive income, net	3	1
Total other comprehensive income attributable to Aon shareholders	3	1
Ending Balance	(8)	3
Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(1,861)	(1,333)
Other comprehensive income (loss) before reclassifications, net	54	(6)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income	0	0
Tax expense	0	0
Amounts reclassified from accumulated other comprehensive income, net	0	0
Total other comprehensive income attributable to Aon shareholders	54	(6)
Ending Balance	(1,807)	(1,339)
Postretirement Benefit Obligation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(2,751)	(2,540)
Other comprehensive income (loss) before reclassifications, net	(1)	10
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income	32	31
Tax expense	(9)	(8)
Amounts reclassified from accumulated other comprehensive income, net	23	23
Total other comprehensive income attributable to Aon shareholders	22	33
Ending Balance	(2,729)	(2,507)
AOCI Attributable to Parent
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(4,623)	(3,871)
Amounts reclassified from accumulated other comprehensive income
Ending Balance	$ (4,544)	$ (3,843)