Subsequent Event - Disposition of Benefits Administration and Business Process Outsourcing (Notes)	12 Months Ended
Dec. 31, 2016
Subsequent Events [Abstract]
Subsequent Event - Disposition of Benefits Administration and Business Process Outsourcing	Subsequent Event - Disposition of Benefits Administration and Business Process Outsourcing On February 9, 2017, Aon entered into a Purchase Agreement (the “Purchase Agreement”) with Tempo Acquisition, LLC (the “Buyer”). Pursuant to the Purchase Agreement, Aon has agreed to sell its benefits administration and business process outsourcing business (the “Tempo Business”) to the Buyer, an entity formed and controlled by affiliates of The Blackstone Group L.P. (the “Sponsor”), and certain designated purchasers that are direct or indirect subsidiaries of the Buyer. The Tempo Business develops and implements software and services solutions, and provides to clients, health and welfare and retirement benefits administration services, hosted and cloud-based human resources business process outsourcing administration and implementation services, and other related communications consulting services. Subject to the terms and conditions of the Purchase Agreement, the Buyer has agreed to purchase all of the outstanding equity interests of the Tempo Business, plus certain related assets, for a purchase price of (i) $4.3 billion in cash payable at closing, subject to customary adjustments set forth in the Purchase Agreement, and (ii) deferred consideration of up to $500 million, plus the assumption of certain liabilities (the “Transaction”). Under the Purchase Agreement, the deferred consideration is payable in cash in an amount equal to 20% of the incremental cash proceeds realized by the affiliates of the Sponsor and certain other equityholders of the Buyer from a liquidity event if (i) total realized cash proceeds to the Sponsor and such other equityholders over the life of their respective investments upon the closing of the Transaction (inclusive of all interim distributions) exceeds 2.25 times the amount of their equity investments in the Buyer at the closing of the Transaction and (ii) the internal rate of return over the life of their respective investments (inclusive of all interim distributions) exceeds 15%. The completion of the Transaction is subject to customary closing conditions, including, among others: (i) the expiration or termination of the waiting period under the Hart-Scott Rodino Antitrust Improvements Act of 1976, as amended; and (ii) the receipt of all approvals and the filing of all applicable filings under any relevant foreign jurisdictions that are required to be made or obtained as set forth in the Purchase Agreement. The Buyer’s obligations under the Purchase Agreement are not conditioned on receipt of financing; however, the Buyer is not required to complete the Transaction until after the completion of a customary fifteen days consecutive business-day marketing period to commence after the Company has provided required financial information to the Buyer. The Buyer has obtained an equity commitment (“Equity Commitment”) from affiliates of the Sponsor and debt financing commitments (“Debt Commitments”) from BofA Merrill Lynch, Barclays, Credit Suisse, Citigroup, Macquarie, Deutsche Bank, and Morgan Stanley, together with certain of their respective affiliates. The aggregate proceeds of the Equity Commitment and the Debt Commitments will be used by the Buyer (i) to pay the Purchase Price, and (ii) to pay fees and expenses incurred by the Buyer in connection with the Transaction. The Purchase Agreement provides that the Buyer will be required to pay to Aon a $215 million termination fee, together with the reimbursement of certain expenses, if the Purchase Agreement is terminated under certain circumstances. Both Aon and the Buyer have the right to terminate the Purchase Agreement if the closing has not occurred on or before August 9, 2017. An affiliate of the Sponsor has guaranteed the payment of the termination fee to the Company, if and when due, under the Purchase Agreement, together with certain other reimbursement obligations of the Buyer under the Purchase Agreement. Both Aon and the Buyer have agreed to indemnify each other for losses arising from certain breaches of the Purchase Agreement and for certain other liabilities, subject to certain limitations. Aon and the Buyer have made representations and warranties and have agreed to covenants related to the Tempo Business and the Transaction. Between the date of the Purchase Agreement and the closing of the Transaction, Aon has agreed to operate the Tempo Business in the ordinary course substantially as operated immediately prior to the date of the Purchase Agreement and to preserve the goodwill of the suppliers, contractors, licensors, employees, customers, and distributors of, and others having business relations with the Tempo Business. Aon and the Buyer have agreed to enter into certain Transaction-related agreements at the closing, including two commercial agreements, a transition services agreement, certain intellectual property license agreements, sub-leases and other customary agreements. Aon will continue to be a significant client of the Tempo Business and the Tempo Business has agreed to use Aon for its broking and other services. In connection with the Transaction, the Company expects to implement a cost reduction program.

Subsequent Event - Disposition of Benefits Administration and Business Process Outsourcing

On February 9, 2017, Aon entered into a Purchase Agreement (the “Purchase Agreement”) with Tempo Acquisition, LLC (the “Buyer”). Pursuant to the Purchase Agreement, Aon has agreed to sell its benefits administration and business process outsourcing business (the “Tempo Business”) to the Buyer, an entity formed and controlled by affiliates of The Blackstone Group L.P. (the “Sponsor”), and certain designated purchasers that are direct or indirect subsidiaries of the Buyer.

The Tempo Business develops and implements software and services solutions, and provides to clients, health and welfare and retirement benefits administration services, hosted and cloud-based human resources business process outsourcing administration and implementation services, and other related communications consulting services.

Subject to the terms and conditions of the Purchase Agreement, the Buyer has agreed to purchase all of the outstanding equity interests of the Tempo Business, plus certain related assets, for a purchase price of (i) $4.3 billion in cash payable at closing, subject to customary adjustments set forth in the Purchase Agreement, and (ii) deferred consideration of up to $500 million, plus the assumption of certain liabilities (the “Transaction”). Under the Purchase Agreement, the deferred consideration is payable in cash in an amount equal to 20% of the incremental cash proceeds realized by the affiliates of the Sponsor and certain other equityholders of the Buyer from a liquidity event if (i) total realized cash proceeds to the Sponsor and such other equityholders over the life of their respective investments upon the closing of the Transaction (inclusive of all interim distributions) exceeds 2.25 times the amount of their equity investments in the Buyer at the closing of the Transaction and (ii) the internal rate of return over the life of their respective investments (inclusive of all interim distributions) exceeds 15%.

The completion of the Transaction is subject to customary closing conditions, including, among others: (i) the expiration or termination of the waiting period under the Hart-Scott Rodino Antitrust Improvements Act of 1976, as amended; and (ii) the receipt of all approvals and the filing of all applicable filings under any relevant foreign jurisdictions that are required to be made or obtained as set forth in the Purchase Agreement. The Buyer’s obligations under the Purchase Agreement are not conditioned on receipt of financing; however, the Buyer is not required to complete the Transaction until after the completion of a customary fifteen days consecutive business-day marketing period to commence after the Company has provided required financial information to the Buyer. The Buyer has obtained an equity commitment (“Equity Commitment”) from affiliates of the Sponsor and debt financing commitments (“Debt Commitments”) from BofA Merrill Lynch, Barclays, Credit Suisse, Citigroup, Macquarie, Deutsche Bank, and Morgan Stanley, together with certain of their respective affiliates. The aggregate proceeds of the Equity Commitment and the Debt Commitments will be used by the Buyer (i) to pay the Purchase Price, and (ii) to pay fees and expenses incurred by the Buyer in connection with the Transaction.

The Purchase Agreement provides that the Buyer will be required to pay to Aon a $215 million termination fee, together with the reimbursement of certain expenses, if the Purchase Agreement is terminated under certain circumstances. Both Aon and the Buyer have the right to terminate the Purchase Agreement if the closing has not occurred on or before August 9, 2017. An affiliate of the Sponsor has guaranteed the payment of the termination fee to the Company, if and when due, under the Purchase Agreement, together with certain other reimbursement obligations of the Buyer under the Purchase Agreement.

Both Aon and the Buyer have agreed to indemnify each other for losses arising from certain breaches of the Purchase Agreement and for certain other liabilities, subject to certain limitations.

Aon and the Buyer have made representations and warranties and have agreed to covenants related to the Tempo Business and the Transaction. Between the date of the Purchase Agreement and the closing of the Transaction, Aon has agreed to operate the Tempo Business in the ordinary course substantially as operated immediately prior to the date of the Purchase Agreement and to preserve the goodwill of the suppliers, contractors, licensors, employees, customers, and distributors of, and others having business relations with the Tempo Business.

Aon and the Buyer have agreed to enter into certain Transaction-related agreements at the closing, including two commercial agreements, a transition services agreement, certain intellectual property license agreements, sub-leases and other customary agreements. Aon will continue to be a significant client of the Tempo Business and the Tempo Business has agreed to use Aon for its broking and other services.

In connection with the Transaction, the Company expects to implement a cost reduction program.