Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenue
Total revenue	$ 2,379			$ 2,349			$ 8,128	$ 8,000
Expenses
Compensation and benefits	1,368			1,392			4,453	4,502
Information technology	120			125			363	363
Premises	76			94			248	283
Depreciation of fixed assets	44			40			124	126
Amortization and impairment of intangible assets	101			100			295	492
Other general expenses	310			336			1,000	1,189
Total operating expenses	2,019			2,087			6,483	6,955
Operating income (loss)	360			262			1,645	1,045
Interest income	1			0			4	5
Interest expense	(78)			(69)			(227)	(208)
Intercompany interest income (expense)	0			0			0	0
Intercompany other income (expense)	0			0			0	0
Other income (expense)	2			1			8	(17)
Income (loss) from continuing operations before income taxes	285			194			1,430	825
Income tax expense (benefit)	56			39			238	9
Net income from continuing operations	229			155			1,192	816
Net income (loss) from discontinued operations	(1)			(2)			(1)	5
Net income (loss) before equity in earnings of subsidiaries	228			153			1,191	821
Equity in earnings of subsidiaries	0			0			0	0
Net income	228	$ 287	$ 676	153	$ 58	$ 610	1,191	821
Less: Net income attributable to noncontrolling interests	6			6			33	32
Net income attributable to Aon shareholders	222			147			1,158	789
Aon plc
Revenue
Total revenue	0			0			0	0
Expenses
Compensation and benefits	12			13			54	47
Information technology	0			0			0	0
Premises	0			0			0	0
Depreciation of fixed assets	0			0			0	0
Amortization and impairment of intangible assets	0			0			0	0
Other general expenses	2			0			5	3
Total operating expenses	14			13			59	50
Operating income (loss)	(14)			(13)			(59)	(50)
Interest income	0			0			0	0
Interest expense	(47)			(52)			(138)	(149)
Intercompany interest income (expense)	4			4			11	11
Intercompany other income (expense)	(70)			245			98	99
Other income (expense)	39			(5)			33	18
Income (loss) from continuing operations before income taxes	(88)			179			(55)	(71)
Income tax expense (benefit)	(2)			(8)			(7)	(27)
Net income from continuing operations	(86)			187			(48)	(44)
Net income (loss) from discontinued operations	0			0			0	0
Net income (loss) before equity in earnings of subsidiaries	(86)			187			(48)	(44)
Equity in earnings of subsidiaries	330			(42)			1,223	846
Net income	244			145			1,175	802
Less: Net income attributable to noncontrolling interests	0			0			0	0
Net income attributable to Aon shareholders	244			145			1,175	802
Aon Corporation
Revenue
Total revenue	0			0			0	0
Expenses
Compensation and benefits	2			3			14	5
Information technology	0			0			0	0
Premises	4			0			17	0
Depreciation of fixed assets	0			0			0	0
Amortization and impairment of intangible assets	0			0			0	0
Other general expenses	1			(1)			3	90
Total operating expenses	7			2			34	95
Operating income (loss)	(7)			(2)			(34)	(95)
Interest income	11			15			29	44
Interest expense	(35)			(26)			(97)	(75)
Intercompany interest income (expense)	(116)			(128)			(348)	(385)
Intercompany other income (expense)	(9)			(245)			(272)	(240)
Other income (expense)	(22)			(3)			(47)	(29)
Income (loss) from continuing operations before income taxes	(178)			(389)			(769)	(780)
Income tax expense (benefit)	(35)			(67)			(141)	(144)
Net income from continuing operations	(143)			(322)			(628)	(636)
Net income (loss) from discontinued operations	0			0			0	0
Net income (loss) before equity in earnings of subsidiaries	(143)			(322)			(628)	(636)
Equity in earnings of subsidiaries	107			(42)			1,132	870
Net income	(36)			(364)			504	234
Less: Net income attributable to noncontrolling interests	0			0			0	0
Net income attributable to Aon shareholders	(36)			(364)			504	234
Other Non-Guarantor Subsidiaries
Revenue
Total revenue	2,379			2,349			8,128	8,000
Expenses
Compensation and benefits	1,354			1,376			4,385	4,450
Information technology	120			125			363	363
Premises	72			94			231	283
Depreciation of fixed assets	44			40			124	126
Amortization and impairment of intangible assets	101			100			295	492
Other general expenses	307			337			992	1,096
Total operating expenses	1,998			2,072			6,390	6,810
Operating income (loss)	381			277			1,738	1,190
Interest income	0			0			0	0
Interest expense	(6)			(6)			(17)	(23)
Intercompany interest income (expense)	112			124			337	374
Intercompany other income (expense)	79			0			174	141
Other income (expense)	7			7			39	7
Income (loss) from continuing operations before income taxes	573			402			2,271	1,689
Income tax expense (benefit)	93			114			386	180
Net income from continuing operations	480			288			1,885	1,509
Net income (loss) from discontinued operations	(1)			(2)			(1)	5
Net income (loss) before equity in earnings of subsidiaries	479			286			1,884	1,514
Equity in earnings of subsidiaries	(36)			(364)			504	234
Net income	443			(78)			2,388	1,748
Less: Net income attributable to noncontrolling interests	6			6			33	32
Net income attributable to Aon shareholders	437			(84)			2,355	1,716
Consolidation Adjustments
Revenue
Total revenue	0			0			0	0
Expenses
Compensation and benefits	0			0			0	0
Information technology	0			0			0	0
Premises	0			0			0	0
Depreciation of fixed assets	0			0			0	0
Amortization and impairment of intangible assets	0			0			0	0
Other general expenses	0			0			0	0
Total operating expenses	0			0			0	0
Operating income (loss)	0			0			0	0
Interest income	(10)			(15)			(25)	(39)
Interest expense	10			15			25	39
Intercompany interest income (expense)	0			0			0	0
Intercompany other income (expense)	0			0			0	0
Other income (expense)	(22)			2			(17)	(13)
Income (loss) from continuing operations before income taxes	(22)			2			(17)	(13)
Income tax expense (benefit)	0			0			0	0
Net income from continuing operations	(22)			2			(17)	(13)
Net income (loss) from discontinued operations	0			0			0	0
Net income (loss) before equity in earnings of subsidiaries	(22)			2			(17)	(13)
Equity in earnings of subsidiaries	(401)			448			(2,859)	(1,950)
Net income	(423)			450			(2,876)	(1,963)
Less: Net income attributable to noncontrolling interests	0			0			0	0
Net income attributable to Aon shareholders	$ (423)			$ 450			$ (2,876)	$ (1,963)