Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue
Total revenue	$ 3,090	$ 2,381
Expenses
Compensation and benefits	1,616	1,469
Information technology	115	88
Premises	93	84
Depreciation of fixed assets	39	54
Amortization and impairment of intangible assets	110	43
Other general expenses	318	308
Total operating expenses	2,291	2,046
Operating income	799	335
Interest income	4	2
Interest expense	(70)	(70)
Intercompany interest income (expense)	0	0
Intercompany other income (expense)	0	0
Other income (expense)	(15)	(2)
Income from continuing operations before income taxes	718	265
Income tax expense (benefit)	114	0
Net income from continuing operations	604	265
Net income from discontinued operations	6	40
Net income (loss) before equity in earnings of subsidiaries	610	305
Equity in earnings of subsidiaries, net of tax	0	0
Net income	610	305
Less: Net income attributable to noncontrolling interests	16	14
Net income attributable to Aon shareholders	594	291
Aon plc
Revenue
Total revenue	0	0
Expenses
Compensation and benefits	19	52
Information technology	0	0
Premises	0	0
Depreciation of fixed assets	0	0
Amortization and impairment of intangible assets	0	0
Other general expenses	1	5
Total operating expenses	20	57
Operating income	(20)	(57)
Interest income	0	0
Interest expense	(49)	(45)
Intercompany interest income (expense)	4	3
Intercompany other income (expense)	(71)	(50)
Other income (expense)	(7)	(10)
Income from continuing operations before income taxes	(143)	(159)
Income tax expense (benefit)	(16)	(14)
Net income from continuing operations	(127)	(145)
Net income from discontinued operations	0	0
Net income (loss) before equity in earnings of subsidiaries	(127)	(145)
Equity in earnings of subsidiaries, net of tax	718	428
Net income	591	283
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	591	283
Aon Corporation
Revenue
Total revenue	0	0
Expenses
Compensation and benefits	1	3
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	2
Total operating expenses	1	5
Operating income	(1)	(5)
Interest income	14	6
Interest expense	(24)	(24)
Intercompany interest income (expense)	(128)	(136)
Intercompany other income (expense)	(5)	7
Other income (expense)	(6)	9
Income from continuing operations before income taxes	(150)	(143)
Income tax expense (benefit)	(27)	(54)
Net income from continuing operations	(123)	(89)
Net income from discontinued operations	0	0
Net income (loss) before equity in earnings of subsidiaries	(123)	(89)
Equity in earnings of subsidiaries, net of tax	704	271
Net income	581	182
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	581	182
Other Non-Guarantor Subsidiaries
Revenue
Total revenue	3,090	2,381
Expenses
Compensation and benefits	1,596	1,414
Information technology	115	88
Premises	93	84
Depreciation of fixed assets	39	54
Amortization and impairment of intangible assets	110	43
Other general expenses	317	301
Total operating expenses	2,270	1,984
Operating income	820	397
Interest income	0	(2)
Interest expense	(7)	(3)
Intercompany interest income (expense)	124	133
Intercompany other income (expense)	76	43
Other income (expense)	(5)	(9)
Income from continuing operations before income taxes	1,008	559
Income tax expense (benefit)	157	68
Net income from continuing operations	851	491
Net income from discontinued operations	6	40
Net income (loss) before equity in earnings of subsidiaries	857	531
Equity in earnings of subsidiaries, net of tax	581	182
Net income	1,438	713
Less: Net income attributable to noncontrolling interests	16	14
Net income attributable to Aon shareholders	1,422	699
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	(10)	(2)
Interest expense	10	2
Intercompany interest income (expense)	0	0
Intercompany other income (expense)	0	0
Other income (expense)	3	8
Income from continuing operations before income taxes	3	8
Income tax expense (benefit)	0	0
Net income from continuing operations	3	8
Net income from discontinued operations	0	0
Net income (loss) before equity in earnings of subsidiaries	3	8
Equity in earnings of subsidiaries, net of tax	(2,003)	(881)
Net income	(2,000)	(873)
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	$ (2,000)	$ (873)