Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Revenue
Commissions, fees and other	$ 2,740	$ 2,736	$ 8,288	$ 8,378
Fiduciary investment income	6	6	16	16
Total revenue	2,746	2,742	8,304	8,394
Expenses
Compensation and benefits	1,611	1,644	4,948	4,980
Other general expenses	713	685	2,079	2,283
Total operating expenses	2,324	2,329	7,027	7,263
Operating income	422	413	1,277	1,131
Interest income	1	3	6	10
Interest expense	(70)	(72)	(212)	(205)
Intercompany interest income (expense)	0	0	0	0
Intercompany other income (expense)	0	0	0	0
Other income	9	8	27	51
Income before income taxes	362	352	1,098	987
Income tax expense (benefit)	48	49	177	155
Income (loss) before equity in earnings of subsidiaries	314	303	921	832
Equity in earnings (loss) of subsidiaries, net of tax	0	0	0	0
Net income	314	303	921	832
Less: Net income attributable to noncontrolling interests	7	8	27	31
Net income attributable to Aon shareholders	307	295	894	801
Aon plc [Member]
Revenue
Commissions, fees and other	0	0	0	0
Fiduciary investment income	0	0	0	0
Total revenue	0	0	0	0
Expenses
Compensation and benefits	25	28	76	85
Other general expenses	(1)	1	5	10
Total operating expenses	24	29	81	95
Operating income	(24)	(29)	(81)	(95)
Interest income	0	(5)	0	(14)
Interest expense	(51)	(33)	(145)	(87)
Intercompany interest income (expense)	3	119	10	358
Intercompany other income (expense)	328	(57)	217	(162)
Other income	(5)	0	(3)	0
Income before income taxes	251	(5)	(2)	0
Income tax expense (benefit)	14	(1)	(33)	0
Income (loss) before equity in earnings of subsidiaries	237	(4)	31	0
Equity in earnings (loss) of subsidiaries, net of tax	67	299	864	801
Net income	304	295	895	801
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	304	295	895	801
Aon Corporation [Member]
Revenue
Commissions, fees and other	0	0	0	0
Fiduciary investment income	0	0	0	0
Total revenue	0	0	0	0
Expenses
Compensation and benefits	4	8	10	29
Other general expenses	3	3	7	6
Total operating expenses	7	11	17	35
Operating income	(7)	(11)	(17)	(35)
Interest income	4	4	13	10
Interest expense	(24)	(34)	(78)	(101)
Intercompany interest income (expense)	(135)	(128)	(405)	(349)
Intercompany other income (expense)	(277)	(17)	(292)	(40)
Other income	1	2	(8)	10
Income before income taxes	(438)	(184)	(787)	(505)
Income tax expense (benefit)	(93)	(63)	(219)	(180)
Income (loss) before equity in earnings of subsidiaries	(345)	(121)	(568)	(325)
Equity in earnings (loss) of subsidiaries, net of tax	209	355	812	925
Net income	(136)	234	244	600
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	(136)	234	244	600
Other Non-Guarantor Subsidiaries [Member]
Revenue
Commissions, fees and other	2,740	2,736	8,288	8,378
Fiduciary investment income	6	6	16	16
Total revenue	2,746	2,742	8,304	8,394
Expenses
Compensation and benefits	1,582	1,608	4,862	4,866
Other general expenses	711	681	2,067	2,267
Total operating expenses	2,293	2,289	6,929	7,133
Operating income	453	453	1,375	1,261
Interest income	5	4	14	14
Interest expense	(3)	(5)	(10)	(17)
Intercompany interest income (expense)	132	9	395	(9)
Intercompany other income (expense)	(51)	74	75	202
Other income	10	6	39	41
Income before income taxes	546	541	1,888	1,492
Income tax expense (benefit)	127	113	429	335
Income (loss) before equity in earnings of subsidiaries	419	428	1,459	1,157
Equity in earnings (loss) of subsidiaries, net of tax	(136)	234	244	600
Net income	283	662	1,703	1,757
Less: Net income attributable to noncontrolling interests	7	8	27	31
Net income attributable to Aon shareholders	276	654	1,676	1,726
Consolidating Adjustments [Member]
Revenue
Commissions, fees and other	0	0	0	0
Fiduciary investment income	0	0	0	0
Total revenue	0	0	0	0
Expenses
Compensation and benefits	0	0	0	0
Other general expenses	0	0	0	0
Total operating expenses	0	0	0	0
Operating income	0	0	0	0
Interest income	(8)	0	(21)	0
Interest expense	8	0	21	0
Intercompany interest income (expense)	0	0	0	0
Intercompany other income (expense)	0	0	0	0
Other income	3	0	(1)	0
Income before income taxes	3	0	(1)	0
Income tax expense (benefit)	0	0	0	0
Income (loss) before equity in earnings of subsidiaries	3	0	(1)	0
Equity in earnings (loss) of subsidiaries, net of tax	(140)	(888)	(1,920)	(2,326)
Net income	(137)	(888)	(1,921)	(2,326)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	$ (137)	$ (888)	$ (1,921)	$ (2,326)