Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenue
Total revenue	$ 2,606		$ 2,561		$ 5,749	$ 5,651
Expenses
Compensation and benefits	1,501		1,494		3,085	3,110
Information technology	126		123		243	238
Premises	85		96		172	189
Depreciation of fixed assets	40		47		80	86
Amortization and impairment of intangible assets	97		282		194	392
Other general expenses	344		535		690	853
Total operating expenses	2,193		2,577		4,464	4,868
Operating income (loss)	413		(16)		1,285	783
Interest income	1		1		3	5
Interest expense	(77)		(69)		(149)	(139)
Intercompany interest income (expense)	0		0		0	0
Intercompany other income (expense)	0		0		0	0
Other income (expense)	6		(3)		6	(18)
Income (loss) from continuing operations before income taxes	343		(87)		1,145	631
Income tax expense (benefit)	56		(144)		182	(30)
Net income from continuing operations	287		57		963	661
Net income from discontinued operations	0		1		0	7
Net income (loss) before equity in earnings of subsidiaries	287		58		963	668
Equity in earnings of subsidiaries	0		0		0	0
Net income	287	$ 676	58	$ 610	963	668
Less: Net income attributable to noncontrolling interests	10		10		27	26
Net income attributable to Aon shareholders	277		48		936	642
Aon plc
Revenue
Total revenue	0		0		0	0
Expenses
Compensation and benefits	22		15		42	34
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	3		2		3	3
Total operating expenses	25		17		45	37
Operating income (loss)	(25)		(17)		(45)	(37)
Interest income	0		0		0	0
Interest expense	(45)		(48)		(91)	(97)
Intercompany interest income (expense)	3		3		7	7
Intercompany other income (expense)	137		(93)		168	(146)
Other income (expense)	(11)		48		(6)	23
Income (loss) from continuing operations before income taxes	59		(107)		33	(250)
Income tax expense (benefit)	0		(3)		(5)	(19)
Net income from continuing operations	59		(104)		38	(231)
Net income from discontinued operations	0		0		0	0
Net income (loss) before equity in earnings of subsidiaries	59		(104)		38	(231)
Equity in earnings of subsidiaries	211		170		893	888
Net income	270		66		931	657
Less: Net income attributable to noncontrolling interests	0		0		0	0
Net income attributable to Aon shareholders	270		66		931	657
Aon Corporation
Revenue
Total revenue	0		0		0	0
Expenses
Compensation and benefits	4		1		12	2
Information technology	0		0		0	0
Premises	9		0		13	0
Depreciation of fixed assets	0		0		0	0
Amortization and impairment of intangible assets	0		0		0	0
Other general expenses	6		91		2	91
Total operating expenses	19		92		27	93
Operating income (loss)	(19)		(92)		(27)	(93)
Interest income	9		15		18	29
Interest expense	(34)		(25)		(62)	(49)
Intercompany interest income (expense)	(116)		(129)		(232)	(257)
Intercompany other income (expense)	(164)		10		(263)	5
Other income (expense)	(14)		(20)		(25)	(26)
Income (loss) from continuing operations before income taxes	(338)		(241)		(591)	(391)
Income tax expense (benefit)	(64)		(50)		(106)	(77)
Net income from continuing operations	(274)		(191)		(485)	(314)
Net income from discontinued operations	0		0		0	0
Net income (loss) before equity in earnings of subsidiaries	(274)		(191)		(485)	(314)
Equity in earnings of subsidiaries	301		207		1,025	912
Net income	27		16		540	598
Less: Net income attributable to noncontrolling interests	0		0		0	0
Net income attributable to Aon shareholders	27		16		540	598
Other Non-Guarantor Subsidiaries
Revenue
Total revenue	2,606		2,561		5,749	5,651
Expenses
Compensation and benefits	1,475		1,478		3,031	3,074
Information technology	126		123		243	238
Premises	76		96		159	189
Depreciation of fixed assets	40		47		80	86
Amortization and impairment of intangible assets	97		282		194	392
Other general expenses	335		442		685	759
Total operating expenses	2,149		2,468		4,392	4,738
Operating income (loss)	457		93		1,357	913
Interest income	0		0		0	0
Interest expense	(6)		(10)		(11)	(17)
Intercompany interest income (expense)	113		126		225	250
Intercompany other income (expense)	27		83		95	141
Other income (expense)	24		(13)		32	0
Income (loss) from continuing operations before income taxes	615		279		1,698	1,287
Income tax expense (benefit)	120		(91)		293	66
Net income from continuing operations	495		370		1,405	1,221
Net income from discontinued operations	0		1		0	7
Net income (loss) before equity in earnings of subsidiaries	495		371		1,405	1,228
Equity in earnings of subsidiaries	27		16		540	598
Net income	522		387		1,945	1,826
Less: Net income attributable to noncontrolling interests	10		10		27	26
Net income attributable to Aon shareholders	512		377		1,918	1,800
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	(8)		(14)		(15)	(24)
Interest expense	8		14		15	24
Intercompany interest income (expense)	0		0		0	0
Intercompany other income (expense)	0		0		0	0
Other income (expense)	7		(18)		5	(15)
Income (loss) from continuing operations before income taxes	7		(18)		5	(15)
Income tax expense (benefit)	0		0		0	0
Net income from continuing operations	7		(18)		5	(15)
Net income from discontinued operations	0		0		0	0
Net income (loss) before equity in earnings of subsidiaries	7		(18)		5	(15)
Equity in earnings of subsidiaries	(539)		(393)		(2,458)	(2,398)
Net income	(532)		(411)		(2,453)	(2,413)
Less: Net income attributable to noncontrolling interests	0		0		0	0
Net income attributable to Aon shareholders	$ (532)		$ (411)		$ (2,453)	$ (2,413)