Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income	$ 590	$ 303	$ 188	$ 341	$ 467	$ 315	$ 313	$ 336	$ 1,422	$ 1,431	$ 1,148
Less: Net income attributable to noncontrolling interests	6	8	10	13	8	6	9	11	37	34	35
Net income attributable to Aon shareholders	$ 584	$ 295	$ 178	$ 328	$ 459	$ 309	$ 304	$ 325	1,385	1,397	1,113
Other comprehensive (loss) gain, net of tax:
Change in fair value of financial instruments									(8)	4	7
Foreign currency translation adjustments									(442)	(507)	(65)
Post-retirement benefit obligation									155	(260)	293
Total other comprehensive (loss) income									(295)	(763)	235
Less: Other comprehensive loss attributable to noncontrolling interests									(6)	(3)	(1)
Total other comprehensive (loss) income attributable to Aon shareholders									(289)	(760)	236
Comprehensive income attributable to Aon shareholders									$ 1,096	$ 637	$ 1,349