Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	$ 273	$ 298
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	13	3
Purchases of investments	(14)	(1)
Net sales of short-term investments - non-fiduciary	(227)	42
Acquisition of businesses, net of cash acquired	(16)	(21)
Proceeds from sale of businesses	97	41
Capital expenditures	(52)	(62)
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	(199)	2
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(685)	(250)
Advances from (to) affiliates	0	0
Issuance of shares for employee benefit plans	(65)	(114)
Issuance of debt	1,045	870
Repayment of debt	(175)	(686)
Cash dividends to shareholders	(82)	(71)
Noncontrolling interests and other financing activities	(42)	(6)
CASH USED FOR FINANCING ACTIVITIES	(4)	(257)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	11	(39)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	81	4
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	384	374
CASH AND CASH EQUIVALENTS AT END OF PERIOD	465	378
Aon plc [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	(47)	164
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	0	0
Purchases of investments	0	0
Net sales of short-term investments - non-fiduciary	0	0
Acquisition of businesses, net of cash acquired	0	0
Proceeds from sale of businesses	0	0
Capital expenditures	0	0
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(685)	(250)
Advances from (to) affiliates	(46)	157
Issuance of shares for employee benefit plans	(64)	(114)
Issuance of debt	996	114
Repayment of debt	(72)	0
Cash dividends to shareholders	(82)	(71)
Noncontrolling interests and other financing activities	0	0
CASH USED FOR FINANCING ACTIVITIES	47	(164)
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 [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	(37)	(188)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	9	0
Purchases of investments	(5)	(1)
Net sales of short-term investments - non-fiduciary	(244)	(1)
Acquisition of businesses, net of cash acquired	0	0
Proceeds from sale of businesses	0	0
Capital expenditures	0	0
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	(240)	(2)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	(219)	(385)
Issuance of shares for employee benefit plans	0	0
Issuance of debt	50	756
Repayment of debt	(100)	(678)
Cash dividends to shareholders	0	0
Noncontrolling interests and other financing activities	0	0
CASH USED FOR FINANCING ACTIVITIES	(269)	(307)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(546)	(497)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	2,083	2,727
CASH AND CASH EQUIVALENTS AT END OF PERIOD	1,537	2,230
Other Non-Guarantor Subsidiaries [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	357	322
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	4	3
Purchases of investments	(9)	0
Net sales of short-term investments - non-fiduciary	17	43
Acquisition of businesses, net of cash acquired	(16)	(21)
Proceeds from sale of businesses	97	41
Capital expenditures	(52)	(62)
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	41	4
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	(147)	(984)
Issuance of shares for employee benefit plans	(1)	0
Issuance of debt	(1)	0
Repayment of debt	(3)	(8)
Cash dividends to shareholders	0	0
Noncontrolling interests and other financing activities	(42)	(6)
CASH USED FOR FINANCING ACTIVITIES	(194)	(998)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	11	(39)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	215	(711)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	1,242	1,361
CASH AND CASH EQUIVALENTS AT END OF PERIOD	1,457	650
Consolidating Adjustments [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	0	0
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	0	0
Purchases of investments	0	0
Net sales of short-term investments - non-fiduciary	0	0
Acquisition of businesses, net of cash acquired	0	0
Proceeds from sale of businesses	0	0
Capital expenditures	0	0
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	412	1,212
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	412	1,212
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	412	1,212
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	(2,941)	(3,714)
CASH AND CASH EQUIVALENTS AT END OF PERIOD	$ (2,529)	$ (2,502)