residential mortgage-backed securities at december 31 , 2012 , our residential mortgage-backed securities portfolio was comprised of $ 31.4 billion fair value of us government agency-backed securities and $ 6.1 billion fair value of non-agency ( private issuer ) securities . the agency securities are generally collateralized by 1-4 family , conforming , fixed-rate residential mortgages . the non-agency securities are also generally collateralized by 1-4 family residential mortgages