Shareholders' Equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	$ 3,913	$ 4,618	$ 4,219	$ 5,140
Other comprehensive income (loss) before reclassifications, net			(190)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			91
Tax expense			(22)
Amounts reclassified from accumulated other comprehensive income, net			69
Total other comprehensive loss attributable to Aon shareholders	(193)	(108)	(121)	(135)
Ending Balance	3,561	$ 4,328	3,561	$ 4,328
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance			(3,909)
Amounts reclassified from accumulated other comprehensive income
Ending Balance	(4,030)		(4,030)
Change in Fair Value of Financial Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance			(15)
Other comprehensive income (loss) before reclassifications, net			(12)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			12
Tax expense			(3)
Amounts reclassified from accumulated other comprehensive income, net			9
Total other comprehensive loss attributable to Aon shareholders			(3)
Ending Balance	(18)		(18)
Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance			(1,319)
Other comprehensive income (loss) before reclassifications, net			(180)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			0
Tax expense			0
Amounts reclassified from accumulated other comprehensive income, net			0
Total other comprehensive loss attributable to Aon shareholders			(180)
Ending Balance	(1,499)		(1,499)
Postretirement Benefit Obligation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance			(2,575)
Other comprehensive income (loss) before reclassifications, net			2
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income			79
Tax expense			(19)
Amounts reclassified from accumulated other comprehensive income, net			60
Total other comprehensive loss attributable to Aon shareholders			62
Ending Balance	$ (2,513)		$ (2,513)