Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
CURRENT ASSETS :
Cash and cash equivalents	$ 384	$ 374	$ 477	$ 291
Short-term investments	356	394
Receivables, net	2,734	2,815
Fiduciary assets	9,932	11,638
Intercompany receivables	0	0
Other current assets	566	602
Total Current Assets	13,972	15,823
Goodwill	8,448	8,860	8,997
Intangible assets, net	2,180	2,520
Fixed assets, net	765	765
Non-current deferred tax assets	141	144
Intercompany receivables	0	0
Prepaid pension	1,033	933
Other non-current assets	625	727
Investment in subsidiary	0	0
TOTAL ASSETS	27,164	29,772
CURRENT LIABILITIES
Fiduciary liabilities	9,932	11,638
Short-term debt and current portion of long-term debt	562	783
Accounts payable and accrued liabilities	1,772	1,805
Intercompany payables	0	0
Other current liabilities	820	788
Total Current Liabilities	13,086	15,014
Long-term debt	5,175	4,799
Deferred tax liabilities	176	313
Pension, other post-retirement and other post-employment liabilities	1,795	2,141
Intercompany payables	0	0
Other non-current liabilities	769	874
TOTAL LIABILITIES	21,001	23,141
TOTAL AON SHAREHOLDERS’ EQUITY	6,106	6,571
Noncontrolling interests	57	60
TOTAL EQUITY	6,163	6,631	8,195	7,805
TOTAL LIABILITIES AND EQUITY	27,164	29,772
Aon plc [Member]
CURRENT ASSETS :
Cash and cash equivalents	0	0	0	131
Short-term investments	0	0
Receivables, net	1	0
Fiduciary assets	0	0
Intercompany receivables	432	455
Other current assets	3	2
Total Current Assets	436	457
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Non-current deferred tax assets	154	159
Intercompany receivables	375	7,399
Prepaid pension	0	0
Other non-current assets	28	20
Investment in subsidiary	11,804	4,962
TOTAL ASSETS	12,797	12,997
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	0	0
Accounts payable and accrued liabilities	2,988	3,755
Intercompany payables	167	122
Other current liabilities	47	0
Total Current Liabilities	3,202	3,877
Long-term debt	3,482	2,544
Deferred tax liabilities	0	0
Pension, other post-retirement and other post-employment liabilities	0	0
Intercompany payables	0	0
Other non-current liabilities	7	5
TOTAL LIABILITIES	6,691	6,426
TOTAL AON SHAREHOLDERS’ EQUITY	6,106	6,571
Noncontrolling interests	0	0
TOTAL EQUITY	6,106	6,571
TOTAL LIABILITIES AND EQUITY	12,797	12,997
Aon Corporation [Member]
CURRENT ASSETS :
Cash and cash equivalents	2,083	2,727	247	199
Short-term investments	209	165
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	1,950	2,814
Other current assets	218	226
Total Current Assets	4,460	5,932
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Non-current deferred tax assets	558	570
Intercompany receivables	526	600
Prepaid pension	6	6
Other non-current assets	124	121
Investment in subsidiary	16,534	15,200
TOTAL ASSETS	22,208	22,429
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	550	767
Accounts payable and accrued liabilities	45	58
Intercompany payables	9,518	8,960
Other current liabilities	56	49
Total Current Liabilities	10,169	9,834
Long-term debt	1,418	1,917
Deferred tax liabilities	0	0
Pension, other post-retirement and other post-employment liabilities	1,313	1,396
Intercompany payables	8,799	7,277
Other non-current liabilities	140	125
TOTAL LIABILITIES	21,839	20,549
TOTAL AON SHAREHOLDERS’ EQUITY	369	1,880
Noncontrolling interests	0	0
TOTAL EQUITY	369	1,880
TOTAL LIABILITIES AND EQUITY	22,208	22,429
Other Non-Guarantor Subsidiaries [Member]
CURRENT ASSETS :
Cash and cash equivalents	1,242	1,361	1,246	0
Short-term investments	147	229
Receivables, net	2,733	2,815
Fiduciary assets	9,932	11,638
Intercompany receivables	7,957	9,156
Other current assets	347	407
Total Current Assets	22,358	25,606
Goodwill	8,448	8,860
Intangible assets, net	2,180	2,520
Fixed assets, net	765	765
Non-current deferred tax assets	107	113
Intercompany receivables	8,633	111
Prepaid pension	1,027	927
Other non-current assets	557	678
Investment in subsidiary	369	1,880
TOTAL ASSETS	44,444	41,460
CURRENT LIABILITIES
Fiduciary liabilities	9,932	11,638
Short-term debt and current portion of long-term debt	12	16
Accounts payable and accrued liabilities	1,680	1,706
Intercompany payables	654	3,343
Other current liabilities	720	772
Total Current Liabilities	12,998	17,475
Long-term debt	275	338
Deferred tax liabilities	854	1,011
Pension, other post-retirement and other post-employment liabilities	482	745
Intercompany payables	735	833
Other non-current liabilities	705	836
TOTAL LIABILITIES	16,049	21,238
TOTAL AON SHAREHOLDERS’ EQUITY	28,338	20,162
Noncontrolling interests	57	60
TOTAL EQUITY	28,395	20,222
TOTAL LIABILITIES AND EQUITY	44,444	41,460
Consolidating Adjustments [Member]
CURRENT ASSETS :
Cash and cash equivalents	(2,941)	(3,714)	$ (1,016)	$ (39)
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	(10,339)	(12,425)
Other current assets	(2)	(33)
Total Current Assets	(13,282)	(16,172)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Non-current deferred tax assets	(678)	(698)
Intercompany receivables	(9,534)	(8,110)
Prepaid pension	0	0
Other non-current assets	(84)	(92)
Investment in subsidiary	(28,707)	(22,042)
TOTAL ASSETS	(52,285)	(47,114)
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	0	0
Accounts payable and accrued liabilities	(2,941)	(3,714)
Intercompany payables	(10,339)	(12,425)
Other current liabilities	(3)	(33)
Total Current Liabilities	(13,283)	(16,172)
Long-term debt	0	0
Deferred tax liabilities	(678)	(698)
Pension, other post-retirement and other post-employment liabilities	0	0
Intercompany payables	(9,534)	(8,110)
Other non-current liabilities	(83)	(92)
TOTAL LIABILITIES	(23,578)	(25,072)
TOTAL AON SHAREHOLDERS’ EQUITY	(28,707)	(22,042)
Noncontrolling interests	0	0
TOTAL EQUITY	(28,707)	(22,042)
TOTAL LIABILITIES AND EQUITY	$ (52,285)	$ (47,114)