Shareholders' Equity - Components of Accumulated other comprehensive loss, net of related tax (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance	$ 6,059	$ 6,527	$ 8,091
Other comprehensive loss before reclassifications	(791)	(389)	(1,068)
Tax benefit	77	23	221
Other comprehensive loss before reclassifications, net	(714)	(366)	(847)
Amounts reclassified from accumulated other comprehensive loss	332	128	126
Tax benefit	(107)	(51)	(39)
Amounts reclassified from accumulated other comprehensive loss, net	225	77	87
Total other comprehensive loss attributable to Aon shareholders	(489)	(289)	(760)
Balance	5,532	6,059	6,527
Change in Fair Value of Investments
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance	(25)	(17)	(21)
Other comprehensive loss before reclassifications	(25)	(4)	(13)
Tax benefit	6	1	4
Other comprehensive loss before reclassifications, net	(19)	(3)	(9)
Amounts reclassified from accumulated other comprehensive loss	10	11	20
Tax benefit	(3)	(16)	(7)
Amounts reclassified from accumulated other comprehensive loss, net	7	(5)	13
Total other comprehensive loss attributable to Aon shareholders	(12)	(8)	4
Balance	(37)	(25)	(17)
Foreign Currency Translation Adjustments
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance	(771)	(335)	169
Other comprehensive loss before reclassifications	(490)	(467)	(492)
Tax benefit	(3)	31	(12)
Other comprehensive loss before reclassifications, net	(493)	(436)	(504)
Amounts reclassified from accumulated other comprehensive loss	0	0	0
Tax benefit	0	0	0
Amounts reclassified from accumulated other comprehensive loss, net	0	0	0
Total other comprehensive loss attributable to Aon shareholders	(493)	(436)	(504)
Balance	(1,264)	(771)	(335)
Post-retirement Benefit Obligations
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance	(2,627)	(2,782)	(2,522)
Other comprehensive loss before reclassifications	(276)	82	(563)
Tax benefit	74	(9)	229
Other comprehensive loss before reclassifications, net	(202)	73	(334)
Amounts reclassified from accumulated other comprehensive loss	322	117	106
Tax benefit	(104)	(35)	(32)
Amounts reclassified from accumulated other comprehensive loss, net	218	82	74
Total other comprehensive loss attributable to Aon shareholders	16	155	(260)
Balance	(2,611)	(2,627)	(2,782)
AOCI Including Portion Attributable to Noncontrolling Interest
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance	(3,423)	(3,134)	(2,374)
Balance	$ (3,912)	$ (3,423)	$ (3,134)