Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Revenue
Total revenue	$ 2,381	$ 2,276
Expenses
Compensation and benefits	1,461	1,345
Information technology	88	83
Premises	84	82
Depreciation of fixed assets	54	38
Amortization of intangible assets	43	37
Other general expenses	308	271
Total operating expenses	2,038	1,856
Operating income	343	420
Interest income	2	2
Interest expense	(70)	(69)
Intercompany interest income (expense)	0	0
Intercompany other (expense) income	0	0
Other income (expense)	(10)	18
Income from continuing operations before income taxes	265	371
Income tax (benefit) expense	0	59
Income from continuing operations	265	312
Income from discontinued operations, net of tax	40	25
(Loss) income before equity in earnings of subsidiaries	305	337
Equity in earnings of subsidiaries, net of tax	0	0
Net income	305	337
Less: Net income attributable to noncontrolling interests	14	12
Net income attributable to Aon shareholders	291	325
Aon plc
Revenue
Total revenue	0	0
Expenses
Compensation and benefits	52	43
Information technology	0	0
Premises	0	0
Depreciation of fixed assets	0	0
Amortization of intangible assets	0	0
Other general expenses	5	7
Total operating expenses	57	50
Operating income	(57)	(50)
Interest income	0	0
Interest expense	(45)	(45)
Intercompany interest income (expense)	3	4
Intercompany other (expense) income	(50)	(54)
Other income (expense)	(10)	0
Income from continuing operations before income taxes	(159)	(145)
Income tax (benefit) expense	(14)	(26)
Income from continuing operations	(145)	(119)
Income from discontinued operations, net of tax	0	0
(Loss) income before equity in earnings of subsidiaries	(145)	(119)
Equity in earnings of subsidiaries, net of tax	428	444
Net income	283	325
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	283	325
Aon Corporation
Revenue
Total revenue	0	0
Expenses
Compensation and benefits	6	3
Information technology	0	0
Premises	0	0
Depreciation of fixed assets	0	0
Amortization of intangible assets	0	0
Other general expenses	2	2
Total operating expenses	8	5
Operating income	(8)	(5)
Interest income	6	5
Interest expense	(24)	(28)
Intercompany interest income (expense)	(136)	(133)
Intercompany other (expense) income	7	1
Other income (expense)	12	(5)
Income from continuing operations before income taxes	(143)	(165)
Income tax (benefit) expense	(54)	(62)
Income from continuing operations	(89)	(103)
Income from discontinued operations, net of tax	0	0
(Loss) income before equity in earnings of subsidiaries	(89)	(103)
Equity in earnings of subsidiaries, net of tax	271	356
Net income	182	253
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	182	253
Other Non-Guarantor Subsidiaries
Revenue
Total revenue	2,381	2,276
Expenses
Compensation and benefits	1,403	1,299
Information technology	88	83
Premises	84	82
Depreciation of fixed assets	54	38
Amortization of intangible assets	43	37
Other general expenses	301	262
Total operating expenses	1,973	1,801
Operating income	408	475
Interest income	(2)	4
Interest expense	(3)	(3)
Intercompany interest income (expense)	133	129
Intercompany other (expense) income	43	53
Other income (expense)	(20)	23
Income from continuing operations before income taxes	559	681
Income tax (benefit) expense	68	147
Income from continuing operations	491	534
Income from discontinued operations, net of tax	40	25
(Loss) income before equity in earnings of subsidiaries	531	559
Equity in earnings of subsidiaries, net of tax	182	253
Net income	713	812
Less: Net income attributable to noncontrolling interests	14	12
Net income attributable to Aon shareholders	699	800
Elimination
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 of intangible assets	0	0
Other general expenses	0	0
Total operating expenses	0	0
Operating income	0	0
Interest income	(2)	(7)
Interest expense	2	7
Intercompany interest income (expense)	0	0
Intercompany other (expense) income	0	0
Other income (expense)	8	0
Income from continuing operations before income taxes	8	0
Income tax (benefit) expense	0	0
Income from continuing operations	8	0
Income from discontinued operations, net of tax	0	0
(Loss) income before equity in earnings of subsidiaries	8	0
Equity in earnings of subsidiaries, net of tax	(881)	(1,053)
Net income	(873)	(1,053)
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	$ (873)	$ (1,053)