Shareholders' Equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	$ 3,583	$ 3,443
Other comprehensive income (loss) before reclassifications, net	(58)	(403)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income	37	35
Tax expense	(9)	(8)
Amounts reclassified from accumulated other comprehensive income, net	28	27
Total other comprehensive income (loss) attributable to Aon shareholders	(30)	(376)
Ending Balance	4,361	3,260
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	10	(9)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income	1	5
Tax expense	0	(1)
Amounts reclassified from accumulated other comprehensive income, net	1	4
Total other comprehensive income (loss) attributable to Aon shareholders	11	(5)
Ending Balance	12	(17)
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	(70)	(395)
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	(70)	(395)
Ending Balance	(1,115)	(1,700)
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	2	1
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income	36	30
Tax expense	(9)	(7)
Amounts reclassified from accumulated other comprehensive income, net	27	23
Total other comprehensive income (loss) attributable to Aon shareholders	29	24
Ending Balance	(2,788)	(2,692)
AOCI Attributable to Parent
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(3,861)	(4,033)
Amounts reclassified from accumulated other comprehensive income
Ending Balance	$ (3,891)	$ (4,409)