Shareholders' Equity - Components of Accumulated other comprehensive loss, net of related tax (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ (3,134)	$ (2,374)	$ (2,610)
Other comprehensive loss before reclassifications	(389)	(1,068)	286
Tax benefit	23	221	(143)
Other comprehensive loss before reclassifications, net	(366)	(847)	143
Amounts reclassified from accumulated other comprehensive loss	128	126	132
Tax benefit	(51)	(39)	(39)
Amounts reclassified from accumulated other comprehensive loss, net	77	87	93
Total other comprehensive (loss) income attributable to Aon shareholders	(289)	(760)	236
Ending balance	(3,423)	(3,134)	(2,374)
Change in Fair Value of Investments [Member]
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(17)	(21)	(28)
Other comprehensive loss before reclassifications	(4)	(13)	15
Tax benefit	1	4	(8)
Other comprehensive loss before reclassifications, net	(3)	(9)	7
Amounts reclassified from accumulated other comprehensive loss	11	20	1
Tax benefit	(16)	(7)	(1)
Amounts reclassified from accumulated other comprehensive loss, net	(5)	13	0
Total other comprehensive (loss) income attributable to Aon shareholders	(8)	4	7
Ending balance	(25)	(17)	(21)
Foreign Currency Translation Adjustments [Member]
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(335)	169	233
Other comprehensive loss before reclassifications	(467)	(492)	(65)
Tax benefit	31	(12)	1
Other comprehensive loss before reclassifications, net	(436)	(504)	(64)
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) income attributable to Aon shareholders	(436)	(504)	(64)
Ending balance	(771)	(335)	169
Post-retirement Benefit Obligations [Member]
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(2,782)	(2,522)	(2,815)
Other comprehensive loss before reclassifications	82	(563)	336
Tax benefit	(9)	229	(136)
Other comprehensive loss before reclassifications, net	73	(334)	200
Amounts reclassified from accumulated other comprehensive loss	117	106	131
Tax benefit	(35)	(32)	(38)
Amounts reclassified from accumulated other comprehensive loss, net	82	74	93
Total other comprehensive (loss) income attributable to Aon shareholders	155	(260)	293
Ending balance	$ (2,627)	$ (2,782)	$ (2,522)