Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 690,000,000	$ 790,000,000
Short-term investments	170,000,000	138,000,000
Receivables, net	3,554,000,000	3,112,000,000
Fiduciary assets	12,401,000,000	11,834,000,000
Current intercompany receivables	0	0
Other current assets	530,000,000	602,000,000
Total current assets	17,345,000,000	16,476,000,000
Goodwill	8,293,000,000	8,165,000,000
Intangible assets, net	746,000,000	783,000,000
Fixed assets, net	666,000,000	621,000,000
Operating lease right-of-use assets	897,000,000	929,000,000
Deferred tax assets	638,000,000	645,000,000
Prepaid pension	1,164,000,000	1,216,000,000
Non-current intercompany receivables	0	0
Other non-current assets	533,000,000	570,000,000
Investment in subsidiary	0	0
Total assets	30,282,000,000	29,405,000,000
Current liabilities
Accounts payable and accrued liabilities	1,549,000,000	1,939,000,000
Short-term debt and current portion of long-term debt	1,884,000,000	712,000,000
Fiduciary liabilities	12,401,000,000	11,834,000,000
Current intercompany payables	0	0
Other current liabilities	1,277,000,000	1,086,000,000
Total current liabilities	17,111,000,000	15,571,000,000
Long-term debt	6,227,000,000	6,627,000,000
Non-current operating lease liabilities	910,000,000	944,000,000
Deferred tax liabilities	189,000,000	199,000,000
Pension, other postretirement, and postemployment liabilities	1,655,000,000	1,738,000,000
Non-current intercompany payables	0	0
Other non-current liabilities	930,000,000	877,000,000
Total liabilities	27,022,000,000	25,956,000,000
Total Aon shareholders’ equity	3,169,000,000	3,375,000,000
Noncontrolling interests	91,000,000	74,000,000
Total equity	3,260,000,000	3,449,000,000	$ 4,862,000,000	$ 4,219,000,000
Total liabilities and equity	30,282,000,000	29,405,000,000
Aon Ireland
Current assets
Cash and cash equivalents	0	0
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Current intercompany receivables	0	0
Other current assets	0	0
Total current assets	0	0
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Operating lease right-of-use assets	0	0
Deferred tax assets	0	0
Prepaid pension	0	0
Non-current intercompany receivables	0	0
Other non-current assets	0	0
Investment in subsidiary	3,169,000,000	3,375,000,000
Total assets	3,169,000,000	3,375,000,000
Current liabilities
Accounts payable and accrued liabilities	0	0
Short-term debt and current portion of long-term debt	0	0
Fiduciary liabilities	0	0
Current intercompany payables	0	0
Other current liabilities	0	0
Total current liabilities	0	0
Long-term debt	0	0
Non-current operating lease liabilities	0	0
Deferred tax liabilities	0	0
Pension, other postretirement, and postemployment liabilities	0	0
Non-current intercompany payables	0	0
Other non-current liabilities	0	0
Total liabilities	0	0
Total Aon shareholders’ equity	3,169,000,000	3,375,000,000
Noncontrolling interests	0	0
Total equity	3,169,000,000	3,375,000,000
Total liabilities and equity	3,169,000,000	3,375,000,000
Aon UK
Current assets
Cash and cash equivalents	0	0
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Current intercompany receivables	208,000,000	246,000,000
Other current assets	0	0
Total current assets	208,000,000	246,000,000
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Operating lease right-of-use assets	0	0
Deferred tax assets	89,000,000	89,000,000
Prepaid pension	0	0
Non-current intercompany receivables	397,000,000	868,000,000
Other non-current assets	0	0
Investment in subsidiary	8,650,000,000	8,899,000,000
Total assets	9,344,000,000	10,102,000,000
Current liabilities
Accounts payable and accrued liabilities	1,345,000,000	2,157,000,000
Short-term debt and current portion of long-term debt	696,000,000	112,000,000
Fiduciary liabilities	0	0
Current intercompany payables	310,000,000	234,000,000
Other current liabilities	0	0
Total current liabilities	2,351,000,000	2,503,000,000
Long-term debt	3,822,000,000	4,223,000,000
Non-current operating lease liabilities	0	0
Deferred tax liabilities	0	0
Pension, other postretirement, and postemployment liabilities	0	0
Non-current intercompany payables	0	0
Other non-current liabilities	2,000,000	1,000,000
Total liabilities	6,175,000,000	6,727,000,000
Total Aon shareholders’ equity	3,169,000,000	3,375,000,000
Noncontrolling interests	0	0
Total equity	3,169,000,000	3,375,000,000
Total liabilities and equity	9,344,000,000	10,102,000,000
Aon Global Holdings Limited
Current assets
Cash and cash equivalents	36,000,000	0
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Current intercompany receivables	90,000,000	89,000,000
Other current assets	0	0
Total current assets	126,000,000	89,000,000
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Operating lease right-of-use assets	0	0
Deferred tax assets	4,000,000	4,000,000
Prepaid pension	0	0
Non-current intercompany receivables	0	0
Other non-current assets	0	0
Investment in subsidiary	8,871,000,000	12,211,000,000
Total assets	9,001,000,000	12,304,000,000
Current liabilities
Accounts payable and accrued liabilities	0	1,994,000,000
Short-term debt and current portion of long-term debt	0	0
Fiduciary liabilities	0	0
Current intercompany payables	61,000,000	61,000,000
Other current liabilities	0	0
Total current liabilities	61,000,000	2,055,000,000
Long-term debt	0	0
Non-current operating lease liabilities	0	0
Deferred tax liabilities	0	0
Pension, other postretirement, and postemployment liabilities	0	0
Non-current intercompany payables	0	0
Other non-current liabilities	0	0
Total liabilities	61,000,000	2,055,000,000
Total Aon shareholders’ equity	8,940,000,000	10,249,000,000
Noncontrolling interests	0	0
Total equity	8,940,000,000	10,249,000,000
Total liabilities and equity	9,001,000,000	12,304,000,000
Aon Corporation
Current assets
Cash and cash equivalents	194,000,000	2,271,000,000
Short-term investments	57,000,000	28,000,000
Receivables, net	0	0
Fiduciary assets	0	0
Current intercompany receivables	2,710,000,000	1,214,000,000
Other current assets	4,000,000	7,000,000
Total current assets	2,965,000,000	3,520,000,000
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Operating lease right-of-use assets	103,000,000	110,000,000
Deferred tax assets	589,000,000	577,000,000
Prepaid pension	7,000,000	7,000,000
Non-current intercompany receivables	261,000,000	261,000,000
Other non-current assets	25,000,000	32,000,000
Investment in subsidiary	19,973,000,000	19,470,000,000
Total assets	23,923,000,000	23,977,000,000
Current liabilities
Accounts payable and accrued liabilities	49,000,000	56,000,000
Short-term debt and current portion of long-term debt	1,136,000,000	600,000,000
Fiduciary liabilities	0	0
Current intercompany payables	16,321,000,000	12,978,000,000
Other current liabilities	82,000,000	80,000,000
Total current liabilities	17,588,000,000	13,714,000,000
Long-term debt	2,405,000,000	2,404,000,000
Non-current operating lease liabilities	136,000,000	143,000,000
Deferred tax liabilities	0	0
Pension, other postretirement, and postemployment liabilities	1,287,000,000	1,348,000,000
Non-current intercompany payables	7,210,000,000	7,212,000,000
Other non-current liabilities	115,000,000	113,000,000
Total liabilities	28,741,000,000	24,934,000,000
Total Aon shareholders’ equity	(4,818,000,000)	(957,000,000)
Noncontrolling interests	0	0
Total equity	(4,818,000,000)	(957,000,000)
Total liabilities and equity	23,923,000,000	23,977,000,000
Other Non-Guarantor Subsidiaries
Current assets
Cash and cash equivalents	1,698,000,000	2,619,000,000
Short-term investments	113,000,000	110,000,000
Receivables, net	3,554,000,000	3,112,000,000
Fiduciary assets	12,401,000,000	11,834,000,000
Current intercompany receivables	14,875,000,000	12,710,000,000
Other current assets	526,000,000	595,000,000
Total current assets	33,167,000,000	30,980,000,000
Goodwill	8,293,000,000	8,165,000,000
Intangible assets, net	746,000,000	783,000,000
Fixed assets, net	666,000,000	621,000,000
Operating lease right-of-use assets	794,000,000	819,000,000
Deferred tax assets	150,000,000	165,000,000
Prepaid pension	1,157,000,000	1,209,000,000
Non-current intercompany receivables	7,044,000,000	7,046,000,000
Other non-current assets	508,000,000	538,000,000
Investment in subsidiary	0	0
Total assets	52,525,000,000	50,326,000,000
Current liabilities
Accounts payable and accrued liabilities	1,393,000,000	1,832,000,000
Short-term debt and current portion of long-term debt	52,000,000	0
Fiduciary liabilities	12,401,000,000	11,834,000,000
Current intercompany payables	1,191,000,000	986,000,000
Other current liabilities	1,195,000,000	1,006,000,000
Total current liabilities	16,232,000,000	15,658,000,000
Long-term debt	0	0
Non-current operating lease liabilities	774,000,000	801,000,000
Deferred tax liabilities	383,000,000	389,000,000
Pension, other postretirement, and postemployment liabilities	368,000,000	390,000,000
Non-current intercompany payables	492,000,000	963,000,000
Other non-current liabilities	813,000,000	763,000,000
Total liabilities	19,062,000,000	18,964,000,000
Total Aon shareholders’ equity	33,372,000,000	31,288,000,000
Noncontrolling interests	91,000,000	74,000,000
Total equity	33,463,000,000	31,362,000,000
Total liabilities and equity	52,525,000,000	50,326,000,000
Consolidation Adjustments
Current assets
Cash and cash equivalents	(1,238,000,000)	(4,100,000,000)
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Current intercompany receivables	(17,883,000,000)	(14,259,000,000)
Other current assets	0	0
Total current assets	(19,121,000,000)	(18,359,000,000)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Operating lease right-of-use assets	0	0
Deferred tax assets	(194,000,000)	(190,000,000)
Prepaid pension	0	0
Non-current intercompany receivables	(7,702,000,000)	(8,175,000,000)
Other non-current assets	0	0
Investment in subsidiary	(40,663,000,000)	(43,955,000,000)
Total assets	(67,680,000,000)	(70,679,000,000)
Current liabilities
Accounts payable and accrued liabilities	(1,238,000,000)	(4,100,000,000)
Short-term debt and current portion of long-term debt	0	0
Fiduciary liabilities	0	0
Current intercompany payables	(17,883,000,000)	(14,259,000,000)
Other current liabilities	0	0
Total current liabilities	(19,121,000,000)	(18,359,000,000)
Long-term debt	0	0
Non-current operating lease liabilities	0	0
Deferred tax liabilities	(194,000,000)	(190,000,000)
Pension, other postretirement, and postemployment liabilities	0	0
Non-current intercompany payables	(7,702,000,000)	(8,175,000,000)
Other non-current liabilities	0	0
Total liabilities	(27,017,000,000)	(26,724,000,000)
Total Aon shareholders’ equity	(40,663,000,000)	(43,955,000,000)
Noncontrolling interests	0	0
Total equity	(40,663,000,000)	(43,955,000,000)
Total liabilities and equity	$ (67,680,000,000)	$ (70,679,000,000)