Shareholders' Equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	$ 3,260	$ 4,862	$ 3,449	$ 4,219
Other comprehensive income (loss) before reclassifications, net			(320)	25
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			74	60
Tax expense			(17)	(13)
Amounts reclassified from accumulated other comprehensive income, net			57	47
Total other comprehensive income (loss) attributable to Aon shareholders	113	(97)	(263)	72
Ending Balance	3,695	3,913	3,695	3,913
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			(11)	(8)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			11	8
Tax expense			(2)	(1)
Amounts reclassified from accumulated other comprehensive income, net			9	7
Total other comprehensive income (loss) attributable to Aon shareholders			(2)	(1)
Ending Balance	(14)	(16)	(14)	(16)
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			(308)	28
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			(308)	28
Ending Balance	(1,613)	(1,291)	(1,613)	(1,291)
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			(1)	5
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			63	52
Tax expense			(15)	(12)
Amounts reclassified from accumulated other comprehensive income, net			48	40
Total other comprehensive income (loss) attributable to Aon shareholders			47	45
Ending Balance	(2,669)	(2,530)	(2,669)	(2,530)
AOCI Attributable to Parent
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(4,409)	(3,740)	(4,033)	(3,909)
Amounts reclassified from accumulated other comprehensive income
Ending Balance	$ (4,296)	$ (3,837)	$ (4,296)	$ (3,837)