Debt	3 Months Ended
Mar. 31, 2013
Debt Disclosure [Abstract]
Debt	Debt The Company uses proceeds from the commercial paper market from time to time in order to meet short-term working capital needs. At March 31, 2013, the Company had $383 million commercial paper outstanding as compared to $50 million of commercial paper outstanding at December 31, 2012. The weighted average commercial paper outstanding for the three months ended March 31, 2013 was $162 million. The weighted average interest rate of the commercial paper outstanding for the same period was 0.40%. In addition to commercial paper outstanding, the Company had £20 million ($30 million at March 31, 2013 exchange rates) outstanding on its five-year €650 million ($836 million at March 31, 2013 exchange rates) multi-currency foreign credit facility ("Euro Facility") as compared to no borrowings on its lines of credit at December 31, 2012. On March 8, 2013, the Company issued $90 million aggregate principal amount of 4.250% Notes Due 2042. The 4.250% Notes Due 2042 constitute a further issuance of, and were consolidated to form a single series of debt securities with, the $166 million aggregate principal amount of the 4.250% Notes Due 2042 issued by Aon plc on December 12, 2012 (collectively, the "Original Notes"). The Original Notes were unconditionally guaranteed as to the payment of principal and interest by Aon Corporation. On April 15, 2013, an S-4 registration statement registering $256 million aggregate principal amount of the 4.250% Notes Due 2042 (the "Exchange Notes") under the Securities Act of 1933 as amended (the "Securities Act") was declared effective by the Securities and Exchange Commission. The Exchange Notes are to be exchanged for the Original Notes. The form and terms of the Exchange Notes are substantially identical in all material respects to those of the Original Notes except that the Exchange Notes are registered under the Securities Act and the transfer restrictions, registration rights and related additional interest provisions applicable to the Original Notes do not apply to the Exchange Notes. Like the Original Notes, the Exchange Notes will issued by Aon plc and will be unconditionally guaranteed by Aon Corporation. On April 29, 2013, the Company amended its Euro Facility agreement to add Aon plc as an additional borrower.

Debt

3 Months Ended

Mar. 31, 2013

The Company uses proceeds from the commercial paper market from time to time in order to meet short-term working capital needs. At March 31, 2013, the Company had $383 million commercial paper outstanding as compared to $50 million of commercial paper outstanding at December 31, 2012. The weighted average commercial paper outstanding for the three months ended March 31, 2013 was $162 million. The weighted average interest rate of the commercial paper outstanding for the same period was 0.40%. In addition to commercial paper outstanding, the Company had £20 million ($30 million at March 31, 2013 exchange rates) outstanding on its five-year €650 million ($836 million at March 31, 2013 exchange rates) multi-currency foreign credit facility ("Euro Facility") as compared to no borrowings on its lines of credit at December 31, 2012.

On March 8, 2013, the Company issued $90 million aggregate principal amount of 4.250% Notes Due 2042. The 4.250% Notes Due 2042 constitute a further issuance of, and were consolidated to form a single series of debt securities with, the $166 million aggregate principal amount of the 4.250% Notes Due 2042 issued by Aon plc on December 12, 2012 (collectively, the "Original Notes"). The Original Notes were unconditionally guaranteed as to the payment of principal and interest by Aon Corporation.

On April 15, 2013, an S-4 registration statement registering $256 million aggregate principal amount of the 4.250% Notes Due 2042 (the "Exchange Notes") under the Securities Act of 1933 as amended (the "Securities Act") was declared effective by the Securities and Exchange Commission. The Exchange Notes are to be exchanged for the Original Notes. The form and terms of the Exchange Notes are substantially identical in all material respects to those of the Original Notes except that the Exchange Notes are registered under the Securities Act and the transfer restrictions, registration rights and related additional interest provisions applicable to the Original Notes do not apply to the Exchange Notes. Like the Original Notes, the Exchange Notes will issued by Aon plc and will be unconditionally guaranteed by Aon Corporation.

On April 29, 2013, the Company amended its Euro Facility agreement to add Aon plc as an additional borrower.