Shareholders' Equity (Deficit) - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 58	$ 1,291	$ (429)	$ 1,158
Other comprehensive income (loss) before reclassifications, net			213	(431)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			98	57
Tax benefit (expense)			(26)	(15)
Amounts reclassified from accumulated other comprehensive income, net			72	42
Total other comprehensive income (loss) attributable to Aon shareholders	206	(417)	285	(389)
Ending balance	162	788	162	788
AOCI Attributable to Parent
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(4,544)	(3,843)	(4,623)	(3,871)
Amounts reclassified from accumulated other comprehensive income
Ending balance	(4,338)	(4,260)	(4,338)	(4,260)
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			7	(5)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			6	(4)
Tax benefit (expense)			(2)	1
Amounts reclassified from accumulated other comprehensive income, net			4	(3)
Total other comprehensive income (loss) attributable to Aon shareholders			11	(8)
Ending balance	0	(6)	0	(6)
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			228	(442)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			0	0
Tax benefit (expense)			0	0
Amounts reclassified from accumulated other comprehensive income, net			0	0
Total other comprehensive income (loss) attributable to Aon shareholders			228	(442)
Ending balance	(1,633)	(1,775)	(1,633)	(1,775)
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			(22)	16
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			92	61
Tax benefit (expense)			(24)	(16)
Amounts reclassified from accumulated other comprehensive income, net			68	45
Total other comprehensive income (loss) attributable to Aon shareholders			46	61
Ending balance	$ (2,705)	$ (2,479)	$ (2,705)	$ (2,479)