Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenue
Total revenue	$ 3,143	$ 3,090
Expenses
Compensation and benefits	1,584	1,616
Information technology	117	115
Premises	87	93
Depreciation of fixed assets	40	39
Amortization and impairment of intangible assets	97	110
Other general expenses	346	318
Total operating expenses	2,271	2,291
Operating income	872	799
Interest income	2	4
Interest expense	(72)	(70)
Intercompany interest income (expense)	0	0
Intercompany other income (expense)	0	0
Other income (expense)	0	(15)
Income from continuing operations before income taxes	802	718
Income tax expense (benefit)	126	114
Net income from continuing operations	676	604
Net income from discontinued operations	0	6
Net income (loss) before equity in earnings of subsidiaries	676	610
Equity in earnings of subsidiaries	0	0
Net income	676	610
Less: Net income attributable to noncontrolling interests	17	16
Net income attributable to Aon shareholders	659	594
Aon plc
Revenue
Total revenue	0	0
Expenses
Compensation and benefits	20	19
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	1
Total operating expenses	20	20
Operating income	(20)	(20)
Interest income	0	0
Interest expense	(46)	(49)
Intercompany interest income (expense)	4	4
Intercompany other income (expense)	31	(53)
Other income (expense)	5	(25)
Income from continuing operations before income taxes	(26)	(143)
Income tax expense (benefit)	(5)	(16)
Net income from continuing operations	(21)	(127)
Net income from discontinued operations	0	0
Net income (loss) before equity in earnings of subsidiaries	(21)	(127)
Equity in earnings of subsidiaries	682	718
Net income	661	591
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	661	591
Aon Corporation
Revenue
Total revenue	0	0
Expenses
Compensation and benefits	8	1
Information technology	0	0
Premises	4	0
Depreciation of fixed assets	0	0
Amortization and impairment of intangible assets	0	0
Other general expenses	(4)	0
Total operating expenses	8	1
Operating income	(8)	(1)
Interest income	9	14
Interest expense	(28)	(24)
Intercompany interest income (expense)	(116)	(128)
Intercompany other income (expense)	(99)	(5)
Other income (expense)	(11)	(6)
Income from continuing operations before income taxes	(253)	(150)
Income tax expense (benefit)	(42)	(27)
Net income from continuing operations	(211)	(123)
Net income from discontinued operations	0	0
Net income (loss) before equity in earnings of subsidiaries	(211)	(123)
Equity in earnings of subsidiaries	724	705
Net income	513	582
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	513	582
Other Non-Guarantor Subsidiaries
Revenue
Total revenue	3,143	3,090
Expenses
Compensation and benefits	1,556	1,596
Information technology	117	115
Premises	83	93
Depreciation of fixed assets	40	39
Amortization and impairment of intangible assets	97	110
Other general expenses	350	317
Total operating expenses	2,243	2,270
Operating income	900	820
Interest income	0	0
Interest expense	(5)	(7)
Intercompany interest income (expense)	112	124
Intercompany other income (expense)	68	58
Other income (expense)	8	13
Income from continuing operations before income taxes	1,083	1,008
Income tax expense (benefit)	173	157
Net income from continuing operations	910	851
Net income from discontinued operations	0	6
Net income (loss) before equity in earnings of subsidiaries	910	857
Equity in earnings of subsidiaries	513	582
Net income	1,423	1,439
Less: Net income attributable to noncontrolling interests	17	16
Net income attributable to Aon shareholders	1,406	1,423
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	(7)	(10)
Interest expense	7	10
Intercompany interest income (expense)	0	0
Intercompany other income (expense)	0	0
Other income (expense)	(2)	3
Income from continuing operations before income taxes	(2)	3
Income tax expense (benefit)	0	0
Net income from continuing operations	(2)	3
Net income from discontinued operations	0	0
Net income (loss) before equity in earnings of subsidiaries	(2)	3
Equity in earnings of subsidiaries	(1,919)	(2,005)
Net income	(1,921)	(2,002)
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	$ (1,921)	$ (2,002)