Shareholders' Equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	$ 3,449	$ 4,219
Other comprehensive income (loss) before reclassifications, net	(403)	146
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income	35	31
Tax expense	(8)	(8)
Amounts reclassified from accumulated other comprehensive income, net (3)	27	23
Total other comprehensive income (loss) attributable to Aon shareholders	(376)	169
Ending Balance	3,260	4,862
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(4,033)	(3,909)
Amounts reclassified from accumulated other comprehensive income
Ending Balance	(4,409)	(3,740)
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	(9)	4
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income	5	5
Tax expense	(1)	(2)
Amounts reclassified from accumulated other comprehensive income, net (3)	4	3
Total other comprehensive income (loss) attributable to Aon shareholders	(5)	7
Ending Balance	(17)	(8)
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	(395)	131
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 (3)	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	(395)	131
Ending Balance	(1,700)	(1,188)
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	11
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income	30	26
Tax expense	(7)	(6)
Amounts reclassified from accumulated other comprehensive income, net (3)	23	20
Total other comprehensive income (loss) attributable to Aon shareholders	24	31
Ending Balance	$ (2,692)	$ (2,544)