Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ (12)	$ 192	$ 778	$ 305	$ 459	$ 326	$ 308	$ 337	$ 1,263	$ 1,430	$ 1,422
Less: Net income attributable to noncontrolling interests	7	7	9	14	7	7	8	12	37	34	37
Net income attributable to Aon shareholders	$ (19)	$ 185	$ 769	$ 291	$ 452	$ 319	$ 300	$ 325	1,226	1,396	1,385
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments									12	(12)	(8)
Foreign currency translation adjustments									390	(495)	(442)
Postretirement benefit obligation									19	16	155
Total other comprehensive income (loss)									421	(491)	(295)
Less: Other comprehensive income attributable to noncontrolling interests									5	(2)	(6)
Total other comprehensive income (loss) attributable to Aon shareholders									416	(489)	(289)
Comprehensive income attributable to Aon shareholders									$ 1,642	$ 907	$ 1,096