Shareholders' Equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	$ 4,219	$ 5,140
Other comprehensive income (loss) before reclassifications, net	146
Amounts reclassified from accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss)	31
Tax expense	(8)
Amounts reclassified from accumulated other comprehensive income (loss), net	23
Total other comprehensive income attributable to Aon shareholders	169	306
Ending Balance	4,862	$ 5,385
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(3,909)
Amounts reclassified from accumulated other comprehensive income (loss)
Ending Balance	(3,740)
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	4
Amounts reclassified from accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss)	5
Tax expense	(2)
Amounts reclassified from accumulated other comprehensive income (loss), net	3
Total other comprehensive income attributable to Aon shareholders	7
Ending Balance	(8)
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	131
Amounts reclassified from accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss)	0
Tax expense	0
Amounts reclassified from accumulated other comprehensive income (loss), net	0
Total other comprehensive income attributable to Aon shareholders	131
Ending Balance	(1,188)
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	11
Amounts reclassified from accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss)	26
Tax expense	(6)
Amounts reclassified from accumulated other comprehensive income (loss), net	20
Total other comprehensive income attributable to Aon shareholders	31
Ending Balance	$ (2,544)