Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended								6 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 459	$ 326	$ 308	$ 337	$ 540	$ 311	$ 214	$ 357	$ 645	$ 971	$ 1,430	$ 1,422	$ 1,431
Less: Net income attributable to noncontrolling interests	7	7	8	12	6	8	10	13			34	37	34
Net income attributable to Aon shareholders	$ 452	$ 319	$ 300	$ 325	$ 534	$ 303	$ 204	$ 344			1,396	1,385	1,397
Other comprehensive (loss) income, net of tax:
Change in fair value of financial instruments											(12)	(8)	4
Foreign currency translation adjustments											(495)	(442)	(507)
Post-retirement benefit obligation											16	155	(260)
Total other comprehensive loss											(491)	(295)	(763)
Less: Other comprehensive loss attributable to noncontrolling interests											(2)	(6)	(3)
Total other comprehensive loss attributable to Aon shareholders											(489)	(289)	(760)
Comprehensive income attributable to Aon shareholders											$ 907	$ 1,096	$ 637