Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2014 USD ($)	Jun. 30, 2013 USD ($)	Jun. 30, 2014 Aon plc [Member] USD ($)	Jun. 30, 2013 Aon plc [Member] USD ($)	Jun. 30, 2014 Aon Corporation [Member] USD ($)	Jun. 30, 2013 Aon Corporation [Member] USD ($)	Jun. 30, 2014 Other Non-Guarantor Subsidiaries [Member] USD ($)	Jun. 30, 2013 Other Non-Guarantor Subsidiaries [Member] USD ($)	Jun. 30, 2014 Consolidating Adjustments [Member] USD ($)	Jun. 30, 2013 Consolidating Adjustments [Member] USD ($)
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	$ 333	$ 387	$ 265	$ (23)	$ (215)	$ (89)	$ 283	$ 499	$ 0	$ 0
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	48	23	0	0	37	2	11	21	0	0
Purchases of long-term investments	(14)	(6)	0	0	(14)	(6)	0	0	0	0
Net sales of short-term investments - non-fiduciary	217	29	0	0	27	26	190	3	0	0
Acquisition of businesses, net of cash acquired	(83)	(23)	0	0	0	0	(83)	(23)	0	0
Proceeds from sale of businesses	1	1	0	0	0	0	1	1	0	0
Capital expenditures	(115)	(122)	0	0	0	0	(115)	(122)	0	0
CASH PROVIDED BY (USED FOR) INVESTING ACTIVITIES	54	(98)	0	0	50	22	4	(120)	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(1,250)	(525)	(1,250)	(525)	0	0	0	0	0	0
Advances (to) from affiliates	0	0	(522)	126	99	444	586	(323)	(163)	(247)
Issuance of shares for employee benefit plans	40	57	40	57	0	0	0	0	0	0
Issuance of debt	3,324	2,914	2,169	639	1,161	2,044	(6)	231	0	0
Repayment of debt	(1,745)	(2,607)	(574)	(6)	(1,161)	(2,362)	(10)	(239)	0	0
Deposit with trustee	(681)	0	0		0		(681)		0
Cash dividends to shareholders	(128)	(105)	(128)	(105)	0	0	0	0	0	0
Purchase of shares from noncontrolling interests	1	0	0	0	0	0	1	0	0	0
Dividends paid to noncontrolling interests	(10)	(6)	0	0	0	0	(10)	(6)	0	0
CASH USED FOR FINANCING ACTIVITIES	(449)	(272)	(265)	186	99	126	(120)	(337)	(163)	(247)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	3	(42)	0	0	0	0	3	(42)	0	0
NET DECREASE IN CASH AND CASH EQUIVALENTS	(59)	(25)	0	163	(66)	59	170	0	(163)	(247)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	477	291	0	131	247	199	1,246	0	(1,016)	(39)
CASH AND CASH EQUIVALENTS AT END OF PERIOD	$ 418	$ 266	$ 0	$ 294	$ 181	$ 258	$ 1,416	$ 0	$ (1,179)	$ (286)