Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FORD) OPERATING ACTIVITIES	$ 1,475	$ 1,295
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	31	23
Purchases of investments	(47)	(3)
Net purchases of short-term investments - non-fiduciary	(108)	(9)
Acquisition of businesses, net of cash acquired	(198)	(26)
Proceeds from sale of businesses	104	54
Capital expenditures	(153)	(225)
CASH USED FOR INVESTING ACTIVITIES	(371)	(186)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(1,037)	(1,150)
Advances from (to) affiliates	0	0
Issuance of shares for employee benefit plans	(70)	(148)
Issuance of debt	2,729	3,494
Repayment of debt	(2,308)	(2,860)
Cash dividends to shareholders	(258)	(240)
Noncontrolling interests and other financing activities	(71)	(26)
CASH USED FOR FINANCING ACTIVITIES	(1,015)	(930)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	10	(155)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	99	24
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	384	374
CASH AND CASH EQUIVALENTS AT END OF PERIOD	483	398
Aon plc [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FORD) OPERATING ACTIVITIES	219	251
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
Proceeds from sale of businesses	0	0
Capital expenditures	0	0
CASH USED FOR INVESTING ACTIVITIES	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(1,037)	(1,150)
Advances from (to) affiliates	166	630
Issuance of shares for employee benefit plans	(70)	(148)
Issuance of debt	1,588	768
Repayment of debt	(608)	(111)
Cash dividends to shareholders	(258)	(240)
Noncontrolling interests and other financing activities	0	0
CASH USED FOR FINANCING ACTIVITIES	(219)	(251)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET INCREASE (DECREASE) 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 FORD) OPERATING ACTIVITIES	(664)	(624)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	19	3
Purchases of investments	(25)	(1)
Net purchases of short-term investments - non-fiduciary	(99)	(51)
Acquisition of businesses, net of cash acquired	0	0
Proceeds from sale of businesses	0	0
Capital expenditures	0	0
CASH USED FOR INVESTING ACTIVITIES	(105)	(49)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	356	451
Issuance of shares for employee benefit plans	0	0
Issuance of debt	1,141	2,720
Repayment of debt	(1,692)	(2,735)
Cash dividends to shareholders	0	0
Noncontrolling interests and other financing activities	0	0
CASH USED FOR FINANCING ACTIVITIES	(195)	436
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(964)	(237)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	2,083	2,727
CASH AND CASH EQUIVALENTS AT END OF PERIOD	1,119	2,490
Other Non-Guarantor Subsidiaries [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FORD) OPERATING ACTIVITIES	1,920	1,668
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	12	20
Purchases of investments	(22)	(2)
Net purchases of short-term investments - non-fiduciary	(9)	42
Acquisition of businesses, net of cash acquired	(198)	(26)
Proceeds from sale of businesses	104	54
Capital expenditures	(153)	(225)
CASH USED FOR INVESTING ACTIVITIES	(266)	(137)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	(670)	(1,616)
Issuance of shares for employee benefit plans	0	0
Issuance of debt	0	6
Repayment of debt	(8)	(14)
Cash dividends to shareholders	0	0
Noncontrolling interests and other financing activities	(71)	(26)
CASH USED FOR FINANCING ACTIVITIES	(749)	(1,650)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	10	(155)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	915	(274)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	1,242	1,361
CASH AND CASH EQUIVALENTS AT END OF PERIOD	2,157	1,087
Consolidating Adjustments [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FORD) OPERATING ACTIVITIES	0	0
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
Proceeds from sale of businesses	0	0
Capital expenditures	0	0
CASH USED FOR INVESTING ACTIVITIES	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	148	535
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	148	535
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	148	535
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	(2,941)	(3,714)
CASH AND CASH EQUIVALENTS AT END OF PERIOD	$ (2,793)	$ (3,179)