19. Commitments and contingencies ( a ) C oncentrations of credit ri sk The Company's investments are managed following prudent standards of diversification. The Company attempts to limit its credit exposure by purchasing high quality fixed income investments to maintain an average portfolio credit quality of AA- or higher with mortgage and commercial mortgage-backed issues having an aggregate weighted average credit quality of AA A. In addition, the Company limits its exposure to any single issuer to 3% or less, excluding government and agency securities. With the exception of the Company's bank loan portfolio, t he minimum credit rating of any security purchased is Baa3 / BBB- and where investments are downgraded, the Company permits a holding of up to 2% in aggregate market value, or 10% with written pre-authorization. At December 31, 2011 , 1.2% of the portfolio , excluding bank loans, had a split rating below Baa3/BBB- and the Company did not have an aggregate exposure to any single issuer of more than 1.0% of its investment portfolio , other than with respect to government and agency securities. The Company underwrites a significant amount of its reinsurance business through three particular brokers as set out below. C redit risk exists should any of these brokers be unable to fulfill their contractual obligations with re s pect to the payments of insurance and rein surance balances to the Company . These companies are large, well established, and there are no indications that any of them are financially troubled. No other broker and no one insured or reinsured accounted for more than 10% of gross premiums written for the periods mentioned. The following table shows the percentage of gross premiums written by broker for the years ended December 31, 2011, 2010 and 2009: Years Ended December 31, December 31, December 31, 2011 2010 2009 Marsh &amp; McLennan 25.9% 25.5% 23.3% Willis Group Holdings Ltd. 17.0% 17.1% 14.0% Aon Benfield Group Ltd. 23.8% 22.6% 25.7% (b) E mployment agreement s The Company has entere