Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CASH FLOWS FROM OPERATING ACTIVITIES
Cash provided by (used for) operating activities - continuing operations	$ 669.0	$ 1,829.0	$ 1,502.0
Cash provided by operating activities - discontinued operations	65.0	497.0	507.0
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	734.0	2,326.0	2,009.0
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	68.0	43.0	220.0
Payments for investments	(64.0)	(64.0)	(266.0)
Net purchases (sales) of short-term investments - non-fiduciary	(232.0)	61.0	9.0
Acquisition of businesses, net of cash acquired	(1,029.0)	(879.0)	(16.0)
Sale of businesses, net of cash sold	4,246.0	107.0	205.0
Capital expenditures	(183.0)	(156.0)	(200.0)
Cash provided by (used for) investing activities - continuing operations	2,806.0	(888.0)	(48.0)
Cash provided by (used for) investing activities - discontinued operations	(19.0)	(66.0)	(90.0)
CASH PROVIDED BY (USED FOR) INVESTING ACTIVITIES	2,787.0	(954.0)	(138.0)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(2,399.0)	(1,257.0)	(1,550.0)
Advances from (to) affiliates	0.0	0.0	0.0
Issuance of shares for employee benefit plans	(121.0)	(129.0)	(30.0)
Issuance of debt	1,654.0	3,467.0	5,351.0
Repayment of debt	(1,999.0)	(2,945.0)	(5,098.0)
Cash dividends to shareholders	(364.0)	(345.0)	(323.0)
Noncontrolling interests and other financing activities	(36.0)	(77.0)	(39.0)
Cash provided by (used for) financing activities - continuing operations	(3,265.0)	(1,286.0)	(1,689.0)
Cash used for financing activities - discontinued operations	0.0	0.0	0.0
CASH USED FOR FINANCING ACTIVITIES	(3,265.0)	(1,286.0)	(1,689.0)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	69.0	(39.0)	(172.0)
NET INCREASE IN CASH AND CASH EQUIVALENTS	325.0	47.0	10.0
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	431.0	384.0	374.0
CASH AND CASH EQUIVALENTS AT END OF YEAR	756.0	431.0	384.0
Cash and cash equivalents of discontinued operations	0.0	5.0	2.0	$ 4.0
Aon plc
CASH FLOWS FROM OPERATING ACTIVITIES
Cash provided by (used for) operating activities - continuing operations	2,787.0	2,705.0	695.0
Cash provided by operating activities - discontinued operations	0.0	0.0	0.0
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	2,787.0	2,705.0	695.0
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	224.0	0.0	0.0
Payments for investments	(261.0)	0.0	(13.0)
Net purchases (sales) of short-term investments - non-fiduciary	0.0	0.0	0.0
Acquisition of businesses, net of cash acquired	0.0	0.0	0.0
Sale of businesses, net of cash sold	0.0	0.0	0.0
Capital expenditures	0.0	0.0	0.0
Cash provided by (used for) investing activities - continuing operations	(37.0)	0.0	(13.0)
Cash provided by (used for) investing activities - discontinued operations	0.0	0.0	0.0
CASH PROVIDED BY (USED FOR) INVESTING ACTIVITIES	(37.0)	0.0	(13.0)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(2,399.0)	(1,257.0)	(1,550.0)
Advances from (to) affiliates	426.0	(2,008.0)	232.0
Issuance of shares for employee benefit plans	(121.0)	(129.0)	(29.0)
Issuance of debt	544.0	1,879.0	1,318.0
Repayment of debt	(835.0)	(845.0)	(330.0)
Cash dividends to shareholders	(364.0)	(345.0)	(323.0)
Noncontrolling interests and other financing activities		0.0	0.0
Cash provided by (used for) financing activities - continuing operations	(2,749.0)	(2,705.0)	(682.0)
Cash used for financing activities - discontinued operations	0.0		0.0
CASH USED FOR FINANCING ACTIVITIES	(2,749.0)	(2,705.0)	(682.0)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0.0	0.0	0.0
NET INCREASE IN CASH AND CASH EQUIVALENTS	1.0	0.0	0.0
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	0.0	0.0	0.0
CASH AND CASH EQUIVALENTS AT END OF YEAR	1.0	0.0	0.0
Aon Corporation
CASH FLOWS FROM OPERATING ACTIVITIES
Cash provided by (used for) operating activities - continuing operations	503.0	(536.0)	464.0
Cash provided by operating activities - discontinued operations	0.0	0.0	0.0
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	503.0	(536.0)	464.0
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	587.0	316.0	27.0
Payments for investments	(29.0)	(35.0)	(47.0)
Net purchases (sales) of short-term investments - non-fiduciary	(215.0)	70.0	(42.0)
Acquisition of businesses, net of cash acquired	0.0	(335.0)	0.0
Sale of businesses, net of cash sold	0.0	0.0	0.0
Capital expenditures	0.0	0.0	0.0
Cash provided by (used for) investing activities - continuing operations	343.0	16.0	(62.0)
Cash provided by (used for) investing activities - discontinued operations	0.0	0.0	0.0
CASH PROVIDED BY (USED FOR) INVESTING ACTIVITIES	343.0	16.0	(62.0)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0.0	0.0	0.0
Advances from (to) affiliates	95.0	570.0	(326.0)
Issuance of shares for employee benefit plans	0.0	0.0	0.0
Issuance of debt	1,100.0	1,588.0	4,026.0
Repayment of debt	(1,150.0)	(2,088.0)	(4,746.0)
Cash dividends to shareholders	0.0	0.0	0.0
Noncontrolling interests and other financing activities	0.0	0.0	0.0
Cash provided by (used for) financing activities - continuing operations	45.0	70.0	(1,046.0)
Cash used for financing activities - discontinued operations	0.0		0.0
CASH USED FOR FINANCING ACTIVITIES	45.0	70.0	(1,046.0)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0.0	0.0	0.0
NET INCREASE IN CASH AND CASH EQUIVALENTS	891.0	(450.0)	(644.0)
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	1,633.0	2,083.0	2,727.0
CASH AND CASH EQUIVALENTS AT END OF YEAR	2,524.0	1,633.0	2,083.0
Other Non-Guarantor Subsidiaries
CASH FLOWS FROM OPERATING ACTIVITIES
Cash provided by (used for) operating activities - continuing operations	2,010.0	2,768.0	2,016.0
Cash provided by operating activities - discontinued operations	65.0	497.0	507.0
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	2,075.0	3,265.0	2,523.0
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	582.0	15.0	193.0
Payments for investments	(576.0)	(29.0)	(219.0)
Net purchases (sales) of short-term investments - non-fiduciary	(17.0)	(9.0)	51.0
Acquisition of businesses, net of cash acquired	(1,029.0)	(608.0)	(16.0)
Sale of businesses, net of cash sold	4,246.0	171.0	205.0
Capital expenditures	(183.0)	(156.0)	(200.0)
Cash provided by (used for) investing activities - continuing operations	3,023.0	(616.0)	14.0
Cash provided by (used for) investing activities - discontinued operations	(19.0)	(66.0)	(90.0)
CASH PROVIDED BY (USED FOR) INVESTING ACTIVITIES	3,004.0	(682.0)	(76.0)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0.0	0.0	0.0
Advances from (to) affiliates	(4,975.0)	(3,037.0)	(2,339.0)
Issuance of shares for employee benefit plans	0.0	0.0	(1.0)
Issuance of debt	10.0	0.0	7.0
Repayment of debt	(14.0)	(12.0)	(22.0)
Cash dividends to shareholders	0.0	0.0	0.0
Noncontrolling interests and other financing activities	(36.0)	(77.0)	(39.0)
Cash provided by (used for) financing activities - continuing operations	(5,015.0)	(3,126.0)	(2,394.0)
Cash used for financing activities - discontinued operations	0.0		0.0
CASH USED FOR FINANCING ACTIVITIES	(5,015.0)	(3,126.0)	(2,394.0)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	69.0	(39.0)	(172.0)
NET INCREASE IN CASH AND CASH EQUIVALENTS	133.0	(582.0)	(119.0)
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	660.0	1,242.0	1,361.0
CASH AND CASH EQUIVALENTS AT END OF YEAR	793.0	660.0	1,242.0
Consolidating Adjustments
CASH FLOWS FROM OPERATING ACTIVITIES
Cash provided by (used for) operating activities - continuing operations	(4,631.0)	(3,108.0)	(1,673.0)
Cash provided by operating activities - discontinued operations	0.0	0.0	0.0
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	(4,631.0)	(3,108.0)	(1,673.0)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	(1,325.0)	(288.0)	0.0
Payments for investments	802.0	0.0	13.0
Net purchases (sales) of short-term investments - non-fiduciary	0.0	0.0	0.0
Acquisition of businesses, net of cash acquired	0.0	64.0	0.0
Sale of businesses, net of cash sold	0.0	(64.0)	0.0
Capital expenditures	0.0	0.0	0.0
Cash provided by (used for) investing activities - continuing operations	(523.0)	(288.0)	13.0
Cash provided by (used for) investing activities - discontinued operations	0.0	0.0	0.0
CASH PROVIDED BY (USED FOR) INVESTING ACTIVITIES	(523.0)	(288.0)	13.0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0.0	0.0	0.0
Advances from (to) affiliates	4,454.0	4,475.0	2,433.0
Issuance of shares for employee benefit plans	0.0	0.0	0.0
Issuance of debt	0.0	0.0	0.0
Repayment of debt	0.0	0.0	0.0
Cash dividends to shareholders	0.0	0.0	0.0
Noncontrolling interests and other financing activities	0.0	0.0	0.0
Cash provided by (used for) financing activities - continuing operations	4,454.0	4,475.0	2,433.0
Cash used for financing activities - discontinued operations	0.0		0.0
CASH USED FOR FINANCING ACTIVITIES	4,454.0	4,475.0	2,433.0
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0.0	0.0	0.0
NET INCREASE IN CASH AND CASH EQUIVALENTS	(700.0)	1,079.0	773.0
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	(1,862.0)	(2,941.0)	(3,714.0)
CASH AND CASH EQUIVALENTS AT END OF YEAR	$ (2,562.0)	$ (1,862.0)	$ (2,941.0)

The cash inflow or outflow from sales or purchases of marketable securities.

No definition available.

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Namespace Prefix:	aon_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

For the entity and the disposal group, cash includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Entity may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Compensating balance arrangements that do not legally restrict the withdrawal or usage of cash amounts may be reported as Cash and Cash Equivalents, while legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or company statements of intention with regard to particular deposits are not generally reported as cash and cash equivalents.

+ References

No definition available.