Shareholders' Equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	$ 3,695	$ 3,913	$ 3,449	$ 4,219
Other comprehensive income (loss) before reclassifications, net			(147)	(190)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			107	91
Tax expense			(26)	(22)
Amounts reclassified from accumulated other comprehensive income, net			81	69
Total other comprehensive income (loss) attributable to Aon shareholders	197	(193)	(66)	(121)
Ending Balance	3,620	3,561	3,620	3,561
Change in Fair Value of Financial Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance			(12)	(15)
Other comprehensive income (loss) before reclassifications, net			(6)	(12)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			13	12
Tax expense			(3)	(3)
Amounts reclassified from accumulated other comprehensive income, net			10	9
Total other comprehensive income (loss) attributable to Aon shareholders			4	(3)
Ending Balance	(8)	(18)	(8)	(18)
Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance			(1,305)	(1,319)
Other comprehensive income (loss) before reclassifications, net			(137)	(180)
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			(137)	(180)
Ending Balance	(1,442)	(1,499)	(1,442)	(1,499)
Postretirement Benefit Obligation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance			(2,716)	(2,575)
Other comprehensive income (loss) before reclassifications, net			(4)	2
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			94	79
Tax expense			(23)	(19)
Amounts reclassified from accumulated other comprehensive income, net			71	60
Total other comprehensive income (loss) attributable to Aon shareholders			67	62
Ending Balance	(2,649)	(2,513)	(2,649)	(2,513)
AOCI Attributable to Parent
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(4,296)	(3,837)	(4,033)	(3,909)
Amounts reclassified from accumulated other comprehensive income
Ending Balance	$ (4,099)	$ (4,030)	$ (4,099)	$ (4,030)