Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 353	$ 153	$ 58	$ 610	$ (12)	$ 192	$ 778	$ 305	$ 1,174	$ 1,263	$ 1,430
Less: Net income attributable to noncontrolling interests	8	6	10	16	7	7	9	14	40	37	34
Net income attributable to Aon shareholders	$ 345	$ 147	$ 48	$ 594	$ (19)	$ 185	$ 769	$ 291	1,134	1,226	1,396
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments									11	12	(12)
Foreign currency translation adjustments									(444)	390	(495)
Postretirement benefit obligation									17	19	16
Total other comprehensive income (loss)									(416)	421	(491)
Less: Other comprehensive income (loss) attributable to noncontrolling interests									(4)	5	(2)
Total other comprehensive income (loss) attributable to Aon shareholders									(412)	416	(489)
Comprehensive income attributable to Aon shareholders									$ 722	$ 1,642	$ 907