Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Cash provided by (used for) operating activities - continuing operations	$ 1,686	$ 669	$ 1,829
Cash provided by operating activities - discontinued operations	0	65	497
Cash provided by (used for) operating activities	1,686	734	2,326
Cash flows from investing activities
Proceeds from investments	71	68	43
Payments for investments	(80)	(64)	(64)
Net purchases (sales) of short-term investments - non-fiduciary	348	(232)	61
Acquisition of businesses, net of cash acquired	(58)	(1,029)	(879)
Sale of businesses, net of cash sold	(10)	4,246	107
Capital expenditures	(240)	(183)	(156)
Cash provided by (used for) investing activities - continuing operations	31	2,806	(888)
Cash used for investing activities - discontinued operations	0	(19)	(66)
Cash provided by (used for) investing activities	31	2,787	(954)
Cash flows from financing activities
Share repurchase	(1,470)	(2,399)	(1,257)
Advances from (to) affiliates	0	0	0
Issuance of shares for employee benefit plans	(149)	(121)	(129)
Issuance of debt	5,754	1,654	3,467
Repayment of debt	(5,417)	(1,999)	(2,945)
Cash dividends to shareholders	(382)	(364)	(345)
Noncontrolling interests and other financing activities	(35)	(36)	(77)
Cash provided by (used for) financing activities - continuing operations	(1,699)	(3,265)	(1,286)
Cash used for financing activities - discontinued operations	0	0	0
Cash used for financing activities	(1,699)	(3,265)	(1,286)
Effect of exchange rates on cash and cash equivalents	(118)	69	(39)
Net increase (decrease) in cash and cash equivalents	(100)	325	47
Cash and cash equivalents at beginning of period	756	431	384
Cash and cash equivalents at end of period	656	756	431
Cash and cash equivalents of discontinued operations		0	5	$ 2
Aon plc
Cash flows from operating activities
Cash provided by (used for) operating activities - continuing operations	1,575	2,787	2,705
Cash provided by operating activities - discontinued operations	0	0	0
Cash provided by (used for) operating activities	1,575	2,787	2,705
Cash flows from investing activities
Proceeds from investments	0	224	0
Payments for investments	(13)	(261)	0
Net purchases (sales) of short-term investments - non-fiduciary	0	0	0
Acquisition of businesses, net of cash acquired	0	0	0
Sale of businesses, net of cash sold	0	0	0
Capital expenditures	0	0	0
Cash provided by (used for) investing activities - continuing operations	(13)	(37)	0
Cash used for investing activities - discontinued operations	0	0	0
Cash provided by (used for) investing activities	(13)	(37)	0
Cash flows from financing activities
Share repurchase	(1,470)	(2,399)	(1,257)
Advances from (to) affiliates	156	426	(2,008)
Issuance of shares for employee benefit plans	(149)	(121)	(129)
Issuance of debt	1,723	544	1,879
Repayment of debt	(1,441)	(835)	(845)
Cash dividends to shareholders	(382)	(364)	(345)
Noncontrolling interests and other financing activities		0	0
Cash provided by (used for) financing activities - continuing operations	(1,563)	(2,749)	(2,705)
Cash used for financing activities - discontinued operations	0		0
Cash used for financing activities	(1,563)	(2,749)	(2,705)
Effect of exchange rates on cash and cash equivalents	0	0	0
Net increase (decrease) in cash and cash equivalents	(1)	1	0
Cash and cash equivalents at beginning of period	1	0	0
Cash and cash equivalents at end of period		1	0
Aon Corporation
Cash flows from operating activities
Cash provided by (used for) operating activities - continuing operations	3	503	(536)
Cash provided by operating activities - discontinued operations	0	0	0
Cash provided by (used for) operating activities	3	503	(536)
Cash flows from investing activities
Proceeds from investments	24	587	316
Payments for investments	(47)	(29)	(35)
Net purchases (sales) of short-term investments - non-fiduciary	299	(215)	70
Acquisition of businesses, net of cash acquired	0	0	(335)
Sale of businesses, net of cash sold	0	0	0
Capital expenditures	0	0	0
Cash provided by (used for) investing activities - continuing operations	276	343	16
Cash used for investing activities - discontinued operations	0	0	0
Cash provided by (used for) investing activities	276	343	16
Cash flows from financing activities
Share repurchase	0	0	0
Advances from (to) affiliates	(2,291)	95	570
Issuance of shares for employee benefit plans	0	0	0
Issuance of debt	4,028	1,100	1,588
Repayment of debt	(3,678)	(1,150)	(2,088)
Cash dividends to shareholders	0	0	0
Noncontrolling interests and other financing activities	0	0	0
Cash provided by (used for) financing activities - continuing operations	(1,941)	45	70
Cash used for financing activities - discontinued operations	0		0
Cash used for financing activities	(1,941)	45	70
Effect of exchange rates on cash and cash equivalents	0	0	0
Net increase (decrease) in cash and cash equivalents	(1,662)	891	(450)
Cash and cash equivalents at beginning of period	2,524	1,633	2,083
Cash and cash equivalents at end of period	862	2,524	1,633
Other Non-Guarantor Subsidiaries
Cash flows from operating activities
Cash provided by (used for) operating activities - continuing operations	3,608	2,010	2,768
Cash provided by operating activities - discontinued operations	0	65	497
Cash provided by (used for) operating activities	3,608	2,075	3,265
Cash flows from investing activities
Proceeds from investments	955	582	15
Payments for investments	(33)	(576)	(29)
Net purchases (sales) of short-term investments - non-fiduciary	49	(17)	(9)
Acquisition of businesses, net of cash acquired	(58)	(1,029)	(608)
Sale of businesses, net of cash sold	(10)	4,246	171
Capital expenditures	(240)	(183)	(156)
Cash provided by (used for) investing activities - continuing operations	663	3,023	(616)
Cash used for investing activities - discontinued operations	0	(19)	(66)
Cash provided by (used for) investing activities	663	3,004	(682)
Cash flows from financing activities
Share repurchase	0	0	0
Advances from (to) affiliates	(4,041)	(4,975)	(3,037)
Issuance of shares for employee benefit plans	0	0	0
Issuance of debt	3	10	0
Repayment of debt	(298)	(14)	(12)
Cash dividends to shareholders	0	0	0
Noncontrolling interests and other financing activities	(35)	(36)	(77)
Cash provided by (used for) financing activities - continuing operations	(4,371)	(5,015)	(3,126)
Cash used for financing activities - discontinued operations	0		0
Cash used for financing activities	(4,371)	(5,015)	(3,126)
Effect of exchange rates on cash and cash equivalents	(118)	69	(39)
Net increase (decrease) in cash and cash equivalents	(218)	133	(582)
Cash and cash equivalents at beginning of period	793	660	1,242
Cash and cash equivalents at end of period	575	793	660
Consolidating Adjustments
Cash flows from operating activities
Cash provided by (used for) operating activities - continuing operations	(3,500)	(4,631)	(3,108)
Cash provided by operating activities - discontinued operations	0	0	0
Cash provided by (used for) operating activities	(3,500)	(4,631)	(3,108)
Cash flows from investing activities
Proceeds from investments	(908)	(1,325)	(288)
Payments for investments	13	802	0
Net purchases (sales) of short-term investments - non-fiduciary	0	0	0
Acquisition of businesses, net of cash acquired	0	0	64
Sale of businesses, net of cash sold	0	0	(64)
Capital expenditures	0	0	0
Cash provided by (used for) investing activities - continuing operations	(895)	(523)	(288)
Cash used for investing activities - discontinued operations	0	0	0
Cash provided by (used for) investing activities	(895)	(523)	(288)
Cash flows from financing activities
Share repurchase	0	0	0
Advances from (to) affiliates	6,176	4,454	4,475
Issuance of shares for employee benefit plans	0	0	0
Issuance of debt	0	0	0
Repayment of debt	0	0	0
Cash dividends to shareholders	0	0	0
Noncontrolling interests and other financing activities	0	0	0
Cash provided by (used for) financing activities - continuing operations	6,176	4,454	4,475
Cash used for financing activities - discontinued operations	0		0
Cash used for financing activities	6,176	4,454	4,475
Effect of exchange rates on cash and cash equivalents	0	0	0
Net increase (decrease) in cash and cash equivalents	1,781	(700)	1,079
Cash and cash equivalents at beginning of period	(2,562)	(1,862)	(2,941)
Cash and cash equivalents at end of period	$ (781)	$ (2,562)	$ (1,862)