Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenue
Total revenue	$ 3,219	$ 3,143
Expenses
Compensation and benefits	1,522	1,584
Information technology	111	117
Premises	73	87
Depreciation of fixed assets	41	40
Amortization and impairment of intangible assets	97	97
Other general expenses	342	346
Total operating expenses	2,186	2,271
Operating income	1,033	872
Interest income	2	2
Interest expense	(83)	(72)
Intercompany interest income (expense)	0	0
Intercompany other income (expense)	0	0
Other income (expense)	29	0
Income from continuing operations before income taxes	981	802
Income tax expense (benefit)	189	126
Net income from continuing operations	792	676
Net income (loss) from discontinued operations	(1)	0
Net income (loss) before equity in earnings of subsidiaries	791	676
Equity in earnings of subsidiaries	0	0
Net income	791	676
Less: Net income attributable to noncontrolling interests	19	17
Net income attributable to Aon shareholders	772	659
Aon Ireland
Revenue
Total revenue	0	0
Expenses
Compensation and benefits	0	0
Information technology	0	0
Premises	0	0
Depreciation of fixed assets	0	0
Amortization and impairment of intangible assets	0	0
Other general expenses	0	0
Total operating expenses	0	0
Operating income	0	0
Interest income	0	0
Interest expense	0	0
Intercompany interest income (expense)	0	0
Intercompany other income (expense)	0	0
Other income (expense)	0	0
Income from continuing operations before income taxes	0	0
Income tax expense (benefit)	0	0
Net income from continuing operations	0	0
Net income (loss) from discontinued operations	0	0
Net income (loss) before equity in earnings of subsidiaries	0	0
Equity in earnings of subsidiaries	773	0
Net income	773	0
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	773	0
Aon UK
Revenue
Total revenue	0	0
Expenses
Compensation and benefits	12	20
Information technology	0	0
Premises	0	0
Depreciation of fixed assets	0	0
Amortization and impairment of intangible assets	0	0
Other general expenses	26	0
Total operating expenses	38	20
Operating income	(38)	(20)
Interest income	0	0
Interest expense	(54)	(46)
Intercompany interest income (expense)	8	4
Intercompany other income (expense)	81	31
Other income (expense)	0	5
Income from continuing operations before income taxes	(3)	(26)
Income tax expense (benefit)	(17)	(5)
Net income from continuing operations	14	(21)
Net income (loss) from discontinued operations	0	0
Net income (loss) before equity in earnings of subsidiaries	14	(21)
Equity in earnings of subsidiaries	759	682
Net income	773	661
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	773	661
Aon Global Holdings Limited
Revenue
Total revenue	0	0
Expenses
Compensation and benefits	0	0
Information technology	0	0
Premises	0	0
Depreciation of fixed assets	0	0
Amortization and impairment of intangible assets	0	0
Other general expenses	0	0
Total operating expenses	0	0
Operating income	0	0
Interest income	0	0
Interest expense	(11)	(24)
Intercompany interest income (expense)	0	0
Intercompany other income (expense)	0	0
Other income (expense)	0	0
Income from continuing operations before income taxes	(11)	(24)
Income tax expense (benefit)	(2)	(5)
Net income from continuing operations	(9)	(19)
Net income (loss) from discontinued operations	0	0
Net income (loss) before equity in earnings of subsidiaries	(9)	(19)
Equity in earnings of subsidiaries	614	619
Net income	605	600
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	605	600
Aon Corporation
Revenue
Total revenue	0	0
Expenses
Compensation and benefits	(12)	8
Information technology	0	0
Premises	5	4
Depreciation of fixed assets	0	0
Amortization and impairment of intangible assets	0	0
Other general expenses	1	1
Total operating expenses	(6)	13
Operating income	6	(13)
Interest income	10	9
Interest expense	(39)	(28)
Intercompany interest income (expense)	(113)	(116)
Intercompany other income (expense)	(128)	(94)
Other income (expense)	(42)	(11)
Income from continuing operations before income taxes	(306)	(253)
Income tax expense (benefit)	(48)	(42)
Net income from continuing operations	(258)	(211)
Net income (loss) from discontinued operations	0	0
Net income (loss) before equity in earnings of subsidiaries	(258)	(211)
Equity in earnings of subsidiaries	862	724
Net income	604	513
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	604	513
Other Non-Guarantor Subsidiaries
Revenue
Total revenue	3,219	3,143
Expenses
Compensation and benefits	1,522	1,556
Information technology	111	117
Premises	68	83
Depreciation of fixed assets	41	40
Amortization and impairment of intangible assets	97	97
Other general expenses	315	345
Total operating expenses	2,154	2,238
Operating income	1,065	905
Interest income	14	20
Interest expense	(1)	(1)
Intercompany interest income (expense)	105	112
Intercompany other income (expense)	47	63
Other income (expense)	72	8
Income from continuing operations before income taxes	1,302	1,107
Income tax expense (benefit)	256	178
Net income from continuing operations	1,046	929
Net income (loss) from discontinued operations	(1)	0
Net income (loss) before equity in earnings of subsidiaries	1,045	929
Equity in earnings of subsidiaries	0	0
Net income	1,045	929
Less: Net income attributable to noncontrolling interests	19	17
Net income attributable to Aon shareholders	1,026	912
Consolidation Adjustments
Revenue
Total revenue	0	0
Expenses
Compensation and benefits	0	0
Information technology	0	0
Premises	0	0
Depreciation of fixed assets	0	0
Amortization and impairment of intangible assets	0	0
Other general expenses	0	0
Total operating expenses	0	0
Operating income	0	0
Interest income	(22)	(27)
Interest expense	22	27
Intercompany interest income (expense)	0	0
Intercompany other income (expense)	0	0
Other income (expense)	(1)	(2)
Income from continuing operations before income taxes	(1)	(2)
Income tax expense (benefit)	0	0
Net income from continuing operations	(1)	(2)
Net income (loss) from discontinued operations	0	0
Net income (loss) before equity in earnings of subsidiaries	(1)	(2)
Equity in earnings of subsidiaries	(3,008)	(2,025)
Net income	(3,009)	(2,027)
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	$ (3,009)	$ (2,027)