Shareholders' Equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	$ 4,862	$ 5,385	$ 4,219	$ 5,140
Other comprehensive income (loss) before reclassifications, net			25
Amounts reclassified from accumulated other comprehensive income:
Amounts reclassified from accumulated other comprehensive income			60
Tax expense			(13)
Amounts reclassified from accumulated other comprehensive income, net			47
Total other comprehensive income (loss) attributable to Aon shareholders	(97)	(333)	72	(27)
Ending Balance	3,913	$ 4,618	3,913	$ 4,618
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	(3,837)		(3,837)
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			(8)
Amounts reclassified from accumulated other comprehensive income:
Amounts reclassified from accumulated other comprehensive income			8
Tax expense			(1)
Amounts reclassified from accumulated other comprehensive income, net			7
Total other comprehensive income (loss) attributable to Aon shareholders			(1)
Ending Balance	(16)		(16)
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			28
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 income (loss) attributable to Aon shareholders			28
Ending Balance	(1,291)		(1,291)
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			5
Amounts reclassified from accumulated other comprehensive income:
Amounts reclassified from accumulated other comprehensive income			52
Tax expense			(12)
Amounts reclassified from accumulated other comprehensive income, net			40
Total other comprehensive income (loss) attributable to Aon shareholders			45
Ending Balance	$ (2,530)		$ (2,530)