Shareholders' Equity - Components of Accumulated other comprehensive loss, net of related tax (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018	Jan. 01, 2017
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 4,648	$ 5,532	$ 6,059
Adoption of new accounting guidance				$ 492	$ 49
Adjusted Beginning Balance				5,140	$ 5,581
Other comprehensive income (loss) before reclassifications	(576)	195	(791)
Tax benefit (expense)	43	47	77
Other comprehensive income (loss) before reclassifications, net	(533)	242	(714)
Amounts reclassified from accumulated other comprehensive income (loss)	157	227	332
Tax benefit (expense)	(36)	(53)	(107)
Amounts reclassified from accumulated other comprehensive income (loss), net	121	174	225
Total other comprehensive income (loss) attributable to Aon shareholders	(412)	416	(489)
Ending Balance	4,219	4,648	5,532
Change in Fair Value of Investments
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(25)	(37)	(25)
Adoption of new accounting guidance				(1)
Adjusted Beginning Balance				(26)
Other comprehensive income (loss) before reclassifications	(15)	18	(25)
Tax benefit (expense)	18	(3)	6
Other comprehensive income (loss) before reclassifications, net	3	15	(19)
Amounts reclassified from accumulated other comprehensive income (loss)	11	(2)	10
Tax benefit (expense)	(3)	(1)	(3)
Amounts reclassified from accumulated other comprehensive income (loss), net	8	(3)	7
Total other comprehensive income (loss) attributable to Aon shareholders	11	12	(12)
Ending Balance	(15)	(25)	(37)
Foreign Currency Translation Adjustments
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(879)	(1,264)	(771)
Adoption of new accounting guidance				0
Adjusted Beginning Balance				(879)
Other comprehensive income (loss) before reclassifications	(437)	397	(490)
Tax benefit (expense)	(3)	(5)	(3)
Other comprehensive income (loss) before reclassifications, net	(440)	392	(493)
Amounts reclassified from accumulated other comprehensive income (loss)	0	(7)	0
Tax benefit (expense)		0	0
Amounts reclassified from accumulated other comprehensive income (loss), net	0	(7)	0
Total other comprehensive income (loss) attributable to Aon shareholders	(440)	385	(493)
Ending Balance	(1,319)	(879)	(1,264)
Post-retirement Benefit Obligations
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(2,592)	(2,611)	(2,627)
Adoption of new accounting guidance				0
Adjusted Beginning Balance				(2,592)
Other comprehensive income (loss) before reclassifications	(124)	(220)	(276)
Tax benefit (expense)	28	55	74
Other comprehensive income (loss) before reclassifications, net	(96)	(165)	(202)
Amounts reclassified from accumulated other comprehensive income (loss)	146	236	322
Tax benefit (expense)	(33)	(52)	(104)
Amounts reclassified from accumulated other comprehensive income (loss), net	113	184	218
Total other comprehensive income (loss) attributable to Aon shareholders	17	19	16
Ending Balance	(2,575)	(2,592)	(2,611)
AOCI Including Portion Attributable to Noncontrolling Interest
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(3,496)	(3,912)	(3,423)
Adoption of new accounting guidance				(1)
Adjusted Beginning Balance				$ (3,497)
Ending Balance	$ (3,909)	$ (3,496)	$ (3,912)