Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2016
Cash flows from operating activities
Cash provided by (used for) operating activities - continuing operations	$ 975	$ 289
Cash provided by operating activities - discontinued operations	0	64
Cash provided by (used for) operating activities	975	353
Cash flows from investing activities
Proceeds from investments	30	43
Payments for investments	(65)	(55)
Net sales (purchases) of short-term investments - non-fiduciary	356	(1,344)
Acquisition of businesses, net of cash acquired	(50)	(172)
Sale of businesses, net of cash sold	(8)	4,194
Capital expenditures	(179)	(125)
Cash provided by (used for) investing activities - continuing operations	84	2,541
Cash used for investing activities - discontinued operations	0	(19)
Cash provided by investing activities	84	2,522
Cash flows from financing activities
Share repurchase	(1,272)	(1,888)
Advances from (to) affiliates	0	0
Issuance of shares for employee benefit plans	(139)	(118)
Issuance of debt	3,960	1,651
Repayment of debt	(3,498)	(1,998)
Cash dividends to shareholders	(285)	(274)
Noncontrolling interests and other financing activities	(21)	(21)
Cash provided by (used for) financing activities - continuing operations	(1,255)	(2,648)
Cash used for financing activities - discontinued operations	0	0
Cash used for financing activities	(1,255)	(2,648)
Effect of exchange rates on cash and cash equivalents	(76)	91
Net increase (decrease) in cash and cash equivalents	(272)	318
Cash and cash equivalents at beginning of period	756	431
Cash and cash equivalents at end of period	484	749
Cash and cash equivalents from discontinued operations			$ 5
Aon plc
Cash flows from operating activities
Cash provided by (used for) operating activities - continuing operations	(143)	(135)
Cash provided by operating activities - discontinued operations	0	0
Cash provided by (used for) operating activities	(143)	(135)
Cash flows from investing activities
Proceeds from investments	0	0
Payments for investments	(12)	(16)
Net sales (purchases) of short-term investments - non-fiduciary	0	0
Acquisition of businesses, net of cash acquired	0	0
Sale of businesses, net of cash sold	0	0
Capital expenditures	0	0
Cash provided by (used for) investing activities - continuing operations	(12)	(16)
Cash used for investing activities - discontinued operations	0	0
Cash provided by investing activities	(12)	(16)
Cash flows from financing activities
Share repurchase	(1,272)	(1,888)
Advances from (to) affiliates	1,292	2,722
Issuance of shares for employee benefit plans	(139)	(118)
Issuance of debt	1,258	544
Repayment of debt	(700)	(835)
Cash dividends to shareholders	(285)	(274)
Noncontrolling interests and other financing activities	0	0
Cash provided by (used for) financing activities - continuing operations	154	151
Cash used for financing activities - discontinued operations	0	0
Cash used for financing activities	154	151
Effect of exchange rates on cash and cash equivalents	0	0
Net increase (decrease) in cash and cash equivalents	(1)	0
Cash and cash equivalents at beginning of period	1	0
Cash and cash equivalents at end of period	0	0
Aon Corporation
Cash flows from operating activities
Cash provided by (used for) operating activities - continuing operations	479	999
Cash provided by operating activities - discontinued operations	0	0
Cash provided by (used for) operating activities	479	999
Cash flows from investing activities
Proceeds from investments	16	576
Payments for investments	(36)	(25)
Net sales (purchases) of short-term investments - non-fiduciary	308	(1,328)
Acquisition of businesses, net of cash acquired	0	1
Sale of businesses, net of cash sold	0	0
Capital expenditures	0	0
Cash provided by (used for) investing activities - continuing operations	288	(776)
Cash used for investing activities - discontinued operations	0	0
Cash provided by investing activities	288	(776)
Cash flows from financing activities
Share repurchase	0	0
Advances from (to) affiliates	(1,564)	1,304
Issuance of shares for employee benefit plans	0	0
Issuance of debt	2,701	1,100
Repayment of debt	(2,501)	(1,150)
Cash dividends to shareholders	0	0
Noncontrolling interests and other financing activities	0	0
Cash provided by (used for) financing activities - continuing operations	(1,364)	1,254
Cash used for financing activities - discontinued operations	0	0
Cash used for financing activities	(1,364)	1,254
Effect of exchange rates on cash and cash equivalents	0	0
Net increase (decrease) in cash and cash equivalents	(597)	1,477
Cash and cash equivalents at beginning of period	2,524	1,633
Cash and cash equivalents at end of period	1,927	3,110
Other Non-Guarantor Subsidiaries
Cash flows from operating activities
Cash provided by (used for) operating activities - continuing operations	2,258	987
Cash provided by operating activities - discontinued operations	0	64
Cash provided by (used for) operating activities	2,258	1,051
Cash flows from investing activities
Proceeds from investments	14	11
Payments for investments	(29)	(571)
Net sales (purchases) of short-term investments - non-fiduciary	48	(16)
Acquisition of businesses, net of cash acquired	(50)	(173)
Sale of businesses, net of cash sold	(8)	4,194
Capital expenditures	(179)	(125)
Cash provided by (used for) investing activities - continuing operations	(204)	3,320
Cash used for investing activities - discontinued operations	0	(19)
Cash provided by investing activities	(204)	3,301
Cash flows from financing activities
Share repurchase	0	0
Advances from (to) affiliates	(2,054)	(4,274)
Issuance of shares for employee benefit plans	0	0
Issuance of debt	1	7
Repayment of debt	(297)	(13)
Cash dividends to shareholders	0	0
Noncontrolling interests and other financing activities	(21)	(21)
Cash provided by (used for) financing activities - continuing operations	(2,371)	(4,301)
Cash used for financing activities - discontinued operations	0	0
Cash used for financing activities	(2,371)	(4,301)
Effect of exchange rates on cash and cash equivalents	(76)	91
Net increase (decrease) in cash and cash equivalents	(393)	142
Cash and cash equivalents at beginning of period	793	660
Cash and cash equivalents at end of period	400	802
Consolidation Adjustments
Cash flows from operating activities
Cash provided by (used for) operating activities - continuing operations	(1,619)	(1,562)
Cash provided by operating activities - discontinued operations	0	0
Cash provided by (used for) operating activities	(1,619)	(1,562)
Cash flows from investing activities
Proceeds from investments	0	(544)
Payments for investments	12	557
Net sales (purchases) of short-term investments - non-fiduciary	0	0
Acquisition of businesses, net of cash acquired		0
Sale of businesses, net of cash sold	0	0
Capital expenditures	0	0
Cash provided by (used for) investing activities - continuing operations	12	13
Cash used for investing activities - discontinued operations	0	0
Cash provided by investing activities	12	13
Cash flows from financing activities
Share repurchase	0	0
Advances from (to) affiliates	2,326	248
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 provided by (used for) financing activities - continuing operations	2,326	248
Cash used for financing activities - discontinued operations	0	0
Cash used for financing activities	2,326	248
Effect of exchange rates on cash and cash equivalents	0	0
Net increase (decrease) in cash and cash equivalents	719	(1,301)
Cash and cash equivalents at beginning of period	(2,562)	(1,862)
Cash and cash equivalents at end of period	$ (1,843)	$ (3,163)