This excerpt taken from the ACGL 8-K filed Dec 14, 2007.
5.1.3 Minimum Rating: The Obligor shall at all times maintains a minimum AM Best Financial Strength Rating of B++.
5.1.4 Maintenance of Adjusted Collateral Value: The Obligor shall at all times maintain Collateral in the Custodial Account maintained in its name in an amount such that the Adjusted Collateral Value (determined on a daily basis) is not less than the sum of all amounts then outstanding with respect to the sum of the Letter of Credit Obligations and Reimbursement Obligations. The Obligor agrees that if the required Adjusted Collateral Value of the Collateral in the Custodial Account is less than the sum of the Letter of Credit Obligations and the Reimbursement Obligations, the Obligor shall immediately, and in no event no later than 5:00 p.m. (New York time) on the Business Day immediately following the date of notice by any Finance Party, pay to the Custodian the amount of any such deficiency, which payment shall be deposited by the Custodian into the applicable Custodial Account in the form of cash or Investments. At any time, other than after the occurrence and during the continuation of a Default or an Event of Default, the Obligor may substitute Collateral to the extent such substitution arises from normal trade activities within the Custodial Account in accordance with the provisions of Clause 1 of the Security Agreement between the Obligor and the Security Agent.