Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2016
Cash flows from operating activities
Cash provided by (used for) operating activities - continuing operations	$ 413	$ 436
Cash provided by operating activities - discontinued operations	0	64
Cash provided by (used for) operating activities	413	500
Cash flows from investing activities
Proceeds from investments	23	29
Payments for investments	(36)	(32)
Net sales (purchases) of short-term investments - non-fiduciary	352	(2,451)
Acquisition of businesses, net of cash acquired	(50)	(149)
Sale of businesses, net of cash sold	1	4,193
Capital expenditures	(111)	(82)
Cash provided by (used for) investing activities - continuing operations	179	1,508
Cash used for investing activities - discontinued operations	0	(19)
Cash provided by investing activities	179	1,489
Cash flows from financing activities
Share repurchase	(971)	(1,100)
Advances from (to) affiliates	0	0
Issuance of shares for employee benefit plans	(150)	(139)
Issuance of debt	2,552	1,651
Repayment of debt	(2,027)	(1,990)
Cash dividends to shareholders	(187)	(182)
Noncontrolling interests and other financing activities	(15)	(10)
Cash provided by (used for) financing activities - continuing operations	(798)	(1,770)
Cash used for financing activities - discontinued operations	0	0
Cash used for financing activities	(798)	(1,770)
Effect of exchange rates on cash and cash equivalents	(63)	34
Net increase (decrease) in cash and cash equivalents	(269)	253
Cash and cash equivalents at beginning of period	756	431
Cash and cash equivalents at end of period	487	684
Cash and cash equivalents from discontinued operations			$ 5
Aon plc
Cash flows from operating activities
Cash provided by (used for) operating activities - continuing operations	(126)	(118)
Cash provided by operating activities - discontinued operations	0	0
Cash provided by (used for) operating activities	(126)	(118)
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	(971)	(1,100)
Advances from (to) affiliates	965	1,846
Issuance of shares for employee benefit plans	(150)	(139)
Issuance of debt	752	544
Repayment of debt	(272)	(835)
Cash dividends to shareholders	(187)	(182)
Noncontrolling interests and other financing activities	0	0
Cash provided by (used for) financing activities - continuing operations	137	134
Cash used for financing activities - discontinued operations	0	0
Cash used for financing activities	137	134
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	582	999
Cash provided by operating activities - discontinued operations	0	0
Cash provided by (used for) operating activities	582	999
Cash flows from investing activities
Proceeds from investments	13	567
Payments for investments	(17)	(15)
Net sales (purchases) of short-term investments - non-fiduciary	296	(2,440)
Acquisition of businesses, net of cash acquired	0	2
Sale of businesses, net of cash sold	0	0
Capital expenditures	0	0
Cash provided by (used for) investing activities - continuing operations	292	(1,886)
Cash used for investing activities - discontinued operations	0	0
Cash provided by investing activities	292	(1,886)
Cash flows from financing activities
Share repurchase	0	0
Advances from (to) affiliates	(810)	1,977
Issuance of shares for employee benefit plans	0	0
Issuance of debt	1,800	1,102
Repayment of debt	(1,461)	(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	(471)	1,929
Cash used for financing activities - discontinued operations	0	0
Cash used for financing activities	(471)	1,929
Effect of exchange rates on cash and cash equivalents	0	0
Net increase (decrease) in cash and cash equivalents	403	1,042
Cash and cash equivalents at beginning of period	2,524	1,633
Cash and cash equivalents at end of period	2,927	2,675
Other Non-Guarantor Subsidiaries
Cash flows from operating activities
Cash provided by (used for) operating activities - continuing operations	759	1,056
Cash provided by operating activities - discontinued operations	0	64
Cash provided by (used for) operating activities	759	1,120
Cash flows from investing activities
Proceeds from investments	10	6
Payments for investments	(19)	(558)
Net sales (purchases) of short-term investments - non-fiduciary	56	(11)
Acquisition of businesses, net of cash acquired	(50)	(151)
Sale of businesses, net of cash sold	1	4,193
Capital expenditures	(111)	(82)
Cash provided by (used for) investing activities - continuing operations	(113)	3,397
Cash used for investing activities - discontinued operations	0	(19)
Cash provided by investing activities	(113)	3,378
Cash flows from financing activities
Share repurchase	0	0
Advances from (to) affiliates	(395)	(4,381)
Issuance of shares for employee benefit plans	0	0
Issuance of debt	0	5
Repayment of debt	(294)	(5)
Cash dividends to shareholders	0	0
Noncontrolling interests and other financing activities	(15)	(10)
Cash provided by (used for) financing activities - continuing operations	(704)	(4,391)
Cash used for financing activities - discontinued operations	0	0
Cash used for financing activities	(704)	(4,391)
Effect of exchange rates on cash and cash equivalents	(63)	34
Net increase (decrease) in cash and cash equivalents	(121)	141
Cash and cash equivalents at beginning of period	793	660
Cash and cash equivalents at end of period	672	801
Consolidation Adjustments
Cash flows from operating activities
Cash provided by (used for) operating activities - continuing operations	(802)	(1,501)
Cash provided by operating activities - discontinued operations	0	0
Cash provided by (used for) operating activities	(802)	(1,501)
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	240	558
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	240	558
Cash used for financing activities - discontinued operations	0	0
Cash used for financing activities	240	558
Effect of exchange rates on cash and cash equivalents	0	0
Net increase (decrease) in cash and cash equivalents	(550)	(930)
Cash and cash equivalents at beginning of period	(2,562)	(1,862)
Cash and cash equivalents at end of period	$ (3,112)	$ (2,792)