Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
CASH FLOWS FROM OPERATING ACTIVITIES
Cash provided by operating activities - continuing operations		$ 182	$ 144
Cash provided by operating activities - discontinued operations		58	129
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES		240	273
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments		25	13
Purchases of investments		(9)	(14)
Net purchases of short-term investments - non-fiduciary		94	(227)
Acquisition of businesses, net of cash acquired		(46)	(16)
Sale of businesses, net of cash sold		(2)	97
Capital expenditures		(34)	(37)
Cash provided by (used for) investing activities - continuing operations		28	(184)
Cash used for investing activities - discontinued operations		(15)	(15)
CASH PROVIDED BY (USED FOR) INVESTING ACTIVITIES		13	(199)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase		(126)	(685)
Advances from (to) affiliates		0	0
Issuance of shares for employee benefit plans		(85)	(65)
Issuance of debt		992	1,045
Repayment of debt		(950)	(175)
Cash dividends to shareholders		(87)	(82)
Noncontrolling interests and other financing activities		(2)	(42)
Cash used for financing activities - continuing operations		(258)	(4)
Cash used for financing activities - discontinued operations		0	0
CASH USED FOR FINANCING ACTIVITIES		(258)	(4)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS		25	11
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS		20	81
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD		431	384
CASH AND CASH EQUIVALENTS AT END OF PERIOD	[1]	451	465
Cash and cash equivalents from discontinued operations		18	3
Aon plc
CASH FLOWS FROM OPERATING ACTIVITIES
Cash provided by operating activities - continuing operations		(28)	(47)
Cash provided by operating activities - discontinued operations		0	0
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES		(28)	(47)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments		0	0
Purchases of investments		0	0
Net purchases of short-term investments - non-fiduciary		0	0
Acquisition of businesses, net of cash acquired		0	0
Sale of businesses, net of cash sold		0	0
Capital expenditures		0	0
Cash provided by (used for) investing activities - continuing operations		0	0
Cash used for investing activities - discontinued operations		0	0
CASH PROVIDED BY (USED FOR) INVESTING ACTIVITIES		0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase		(126)	(685)
Advances from (to) affiliates		290	(46)
Issuance of shares for employee benefit plans		(85)	(64)
Issuance of debt		457	996
Repayment of debt		(421)	(72)
Cash dividends to shareholders		(87)	(82)
Noncontrolling interests and other financing activities		0	0
Cash used for financing activities - continuing operations		28	47
Cash used for financing activities - discontinued operations		0	0
CASH USED FOR FINANCING ACTIVITIES		28	47
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS		0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS		0	0
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD		0	0
CASH AND CASH EQUIVALENTS AT END OF PERIOD		0	0
Aon Corporation
CASH FLOWS FROM OPERATING ACTIVITIES
Cash provided by operating activities - continuing operations		1,117	(37)
Cash provided by operating activities - discontinued operations		0	0
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES		1,117	(37)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments		565	9
Purchases of investments		(4)	(5)
Net purchases of short-term investments - non-fiduciary		94	(244)
Acquisition of businesses, net of cash acquired		0	0
Sale of businesses, net of cash sold		0	0
Capital expenditures		0	0
Cash provided by (used for) investing activities - continuing operations		655	(240)
Cash used for investing activities - discontinued operations		0	0
CASH PROVIDED BY (USED FOR) INVESTING ACTIVITIES		655	(240)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase		0	0
Advances from (to) affiliates		(1,990)	(219)
Issuance of shares for employee benefit plans		0	0
Issuance of debt		535	50
Repayment of debt		(525)	(100)
Cash dividends to shareholders		0	0
Noncontrolling interests and other financing activities		0	0
Cash used for financing activities - continuing operations		(1,980)	(269)
Cash used for financing activities - discontinued operations		0	0
CASH USED FOR FINANCING ACTIVITIES		(1,980)	(269)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS		0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS		(208)	(546)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD		1,633	2,083
CASH AND CASH EQUIVALENTS AT END OF PERIOD		1,425	1,537
Other Non-Guarantor Subsidiaries
CASH FLOWS FROM OPERATING ACTIVITIES
Cash provided by operating activities - continuing operations		533	228
Cash provided by operating activities - discontinued operations		58	129
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES		591	357
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments		4	4
Purchases of investments		(546)	(9)
Net purchases of short-term investments - non-fiduciary		0	17
Acquisition of businesses, net of cash acquired		(46)	(16)
Sale of businesses, net of cash sold		(2)	97
Capital expenditures		(34)	(37)
Cash provided by (used for) investing activities - continuing operations		(624)	56
Cash used for investing activities - discontinued operations		(15)	(15)
CASH PROVIDED BY (USED FOR) INVESTING ACTIVITIES		(639)	41
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase		0	0
Advances from (to) affiliates		(17)	(147)
Issuance of shares for employee benefit plans		0	(1)
Issuance of debt		0	(1)
Repayment of debt		(4)	(3)
Cash dividends to shareholders		0	0
Noncontrolling interests and other financing activities		(2)	(42)
Cash used for financing activities - continuing operations		(23)	(194)
Cash used for financing activities - discontinued operations		0	0
CASH USED FOR FINANCING ACTIVITIES		(23)	(194)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS		25	11
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS		(46)	215
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD		660	1,242
CASH AND CASH EQUIVALENTS AT END OF PERIOD		614	1,457
Elimination
CASH FLOWS FROM OPERATING ACTIVITIES
Cash provided by operating activities - continuing operations		(1,440)	0
Cash provided by operating activities - discontinued operations		0	0
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES		(1,440)	0
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments		(544)	0
Purchases of investments		541	0
Net purchases of short-term investments - non-fiduciary		0	0
Acquisition of businesses, net of cash acquired		0	0
Sale of businesses, net of cash sold		0	0
Capital expenditures		0	0
Cash provided by (used for) investing activities - continuing operations		(3)	0
Cash used for investing activities - discontinued operations		0	0
CASH PROVIDED BY (USED FOR) INVESTING ACTIVITIES		(3)	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase		0	0
Advances from (to) affiliates		1,717	412
Issuance of shares for employee benefit plans		0	0
Issuance of debt		0	0
Repayment of debt		0	0
Cash dividends to shareholders		0	0
Noncontrolling interests and other financing activities		0	0
Cash used for financing activities - continuing operations		1,717	412
Cash used for financing activities - discontinued operations		0	0
CASH USED FOR FINANCING ACTIVITIES		1,717	412
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS		0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS		274	412
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD		(1,862)	(2,941)
CASH AND CASH EQUIVALENTS AT END OF PERIOD		$ (1,588)	$ (2,529)

[1] Includes 18 million and $3 million of discontinued operations at March 31, 2017 and March 31, 2016, respectively.