Shareholders' Equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	$ 4,361	$ 3,260	$ 3,583	$ 3,443
Other comprehensive income (loss) before reclassifications, net			62	(320)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			72	74
Tax expense			(17)	(17)
Amounts reclassified from accumulated other comprehensive income, net			55	57
Total other comprehensive income (loss) attributable to Aon shareholders	147	113	117	(263)
Ending Balance	4,567	3,695	4,567	3,695
Change in Fair Value of Financial Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance			1	(12)
Other comprehensive income (loss) before reclassifications, net			9	(11)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			1	11
Tax expense			0	(2)
Amounts reclassified from accumulated other comprehensive income, net			1	9
Total other comprehensive income (loss) attributable to Aon shareholders			10	(2)
Ending Balance	11	(14)	11	(14)
Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance			(1,045)	(1,305)
Other comprehensive income (loss) before reclassifications, net			45	(308)
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 (loss) attributable to Aon shareholders			45	(308)
Ending Balance	(1,000)	(1,613)	(1,000)	(1,613)
Postretirement Benefit Obligation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance			(2,817)	(2,716)
Other comprehensive income (loss) before reclassifications, net			8	(1)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			71	63
Tax expense			(17)	(15)
Amounts reclassified from accumulated other comprehensive income, net			54	48
Total other comprehensive income (loss) attributable to Aon shareholders			62	47
Ending Balance	(2,755)	(2,669)	(2,755)	(2,669)
AOCI Attributable to Parent
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(3,891)	(4,409)	(3,861)	(4,033)
Amounts reclassified from accumulated other comprehensive income
Ending Balance	$ (3,744)	$ (4,296)	$ (3,744)	$ (4,296)