Shareholders' Equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	$ 4,567	$ 3,695	$ 3,583	$ 3,443
Other comprehensive income (loss) before reclassifications, net			(96)	(147)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			105	107
Tax expense			(25)	(26)
Amounts reclassified from accumulated other comprehensive income, net			80	81
Total other comprehensive income (loss) attributable to Aon shareholders	(133)	197	(16)	(66)
Ending Balance	2,299	3,620	2,299	3,620
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			4	(6)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			1	13
Tax expense			0	(3)
Amounts reclassified from accumulated other comprehensive income, net			1	10
Total other comprehensive income (loss) attributable to Aon shareholders			5	4
Ending Balance	6	(8)	6	(8)
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			(108)	(137)
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			(108)	(137)
Ending Balance	(1,153)	(1,442)	(1,153)	(1,442)
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	(4)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			104	94
Tax expense			(25)	(23)
Amounts reclassified from accumulated other comprehensive income, net			79	71
Total other comprehensive income (loss) attributable to Aon shareholders			87	67
Ending Balance	(2,730)	(2,649)	(2,730)	(2,649)
AOCI Attributable to Parent
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(3,744)	(4,296)	(3,861)	(4,033)
Amounts reclassified from accumulated other comprehensive income
Ending Balance	$ (3,877)	$ (4,099)	$ (3,877)	$ (4,099)