Shareholders' Equity - Components of Accumulated other comprehensive loss, net of related tax (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance	$ 5,532	$ 6,059	$ 6,527
Other comprehensive income (loss) before reclassifications	195	(791)	(389)
Tax benefit (expense)	47	77	23
Other comprehensive income (loss) before reclassifications, net	242	(714)	(366)
Amounts reclassified from accumulated other comprehensive income (loss)	227	332	128
Tax benefit (expense)	(53)	(107)	(51)
Amounts reclassified from accumulated other comprehensive income (loss), net	174	225	77
Total other comprehensive income (loss) attributable to Aon shareholders	416	(489)	(289)
Balance	4,648	5,532	6,059
Change in Fair Value of Investments
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance	(37)	(25)	(17)
Other comprehensive income (loss) before reclassifications	18	(25)	(4)
Tax benefit (expense)	(3)	6	1
Other comprehensive income (loss) before reclassifications, net	15	(19)	(3)
Amounts reclassified from accumulated other comprehensive income (loss)	(2)	10	11
Tax benefit (expense)	(1)	(3)	(16)
Amounts reclassified from accumulated other comprehensive income (loss), net	(3)	7	(5)
Total other comprehensive income (loss) attributable to Aon shareholders	12	(12)	(8)
Balance	(25)	(37)	(25)
Foreign Currency Translation Adjustments
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance	(1,264)	(771)	(335)
Other comprehensive income (loss) before reclassifications	397	(490)	(467)
Tax benefit (expense)	(5)	(3)	31
Other comprehensive income (loss) before reclassifications, net	392	(493)	(436)
Amounts reclassified from accumulated other comprehensive income (loss)	(7)	0	0
Tax benefit (expense)	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss), net	(7)	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	385	(493)	(436)
Balance	(879)	(1,264)	(771)
Post-retirement Benefit Obligations
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance	(2,611)	(2,627)	(2,782)
Other comprehensive income (loss) before reclassifications	(220)	(276)	82
Tax benefit (expense)	55	74	(9)
Other comprehensive income (loss) before reclassifications, net	(165)	(202)	73
Amounts reclassified from accumulated other comprehensive income (loss)	236	322	117
Tax benefit (expense)	(52)	(104)	(35)
Amounts reclassified from accumulated other comprehensive income (loss), net	184	218	82
Total other comprehensive income (loss) attributable to Aon shareholders	19	16	155
Balance	(2,592)	(2,611)	(2,627)
AOCI Including Portion Attributable to Noncontrolling Interest
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance	(3,912)	(3,423)	(3,134)
Balance	$ (3,496)	$ (3,912)	$ (3,423)