Shareholders' Equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance			$ 4,648
Adoption of new accounting guidance					$ 492
Balance at January 1, 2018					5,140	$ 5,581
Other comprehensive income (loss) before reclassifications, net			(231)
Amounts reclassified from accumulated other comprehensive loss:
Amounts reclassified from accumulated other comprehensive income			123
Tax expense			(27)
Amounts reclassified from accumulated other comprehensive income, net			96
Total other comprehensive income (loss) attributable to Aon shareholders	$ (108)	$ 265	(135)	$ 500
Ending Balance	4,328	$ 5,247	4,328	$ 5,247
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance			(3,496)
Balance at January 1, 2018					(3,497)
Amounts reclassified from accumulated other comprehensive loss:
Ending Balance	(3,632)		(3,632)
Change in Fair Value of Financial Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance			(25)
Balance at January 1, 2018					(26)
Other comprehensive income (loss) before reclassifications, net			7
Amounts reclassified from accumulated other comprehensive loss:
Amounts reclassified from accumulated other comprehensive income			9
Tax expense			(2)
Amounts reclassified from accumulated other comprehensive income, net			7
Total other comprehensive income (loss) attributable to Aon shareholders			14
Ending Balance	(12)		(12)
Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance			(879)
Balance at January 1, 2018					(879)
Other comprehensive income (loss) before reclassifications, net			(257)
Amounts reclassified from accumulated other comprehensive loss:
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			(257)
Ending Balance	(1,136)		(1,136)
Postretirement Benefit Obligation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance			(2,592)
Balance at January 1, 2018					(2,592)
Other comprehensive income (loss) before reclassifications, net			19
Amounts reclassified from accumulated other comprehensive loss:
Amounts reclassified from accumulated other comprehensive income			114
Tax expense			(25)
Amounts reclassified from accumulated other comprehensive income, net			89
Total other comprehensive income (loss) attributable to Aon shareholders			108
Ending Balance	$ (2,484)		$ (2,484)
Accounting Standards Update 2016-01 | Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adoption of new accounting guidance					(1)
Accounting Standards Update 2016-01 | Change in Fair Value of Financial Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adoption of new accounting guidance					(1)
Accounting Standards Update 2016-01 | Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adoption of new accounting guidance					0
Accounting Standards Update 2016-01 | Postretirement Benefit Obligation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adoption of new accounting guidance					$ 0