187 remained low in 2013 due largely to readily available access to funding and strong corporate balance sheet fundamentals. Commodity  prices were mixed and volatile over the second half of 2013 after trending lower in the first half of 2013 as growth expectations  weakened. The rise in US Treasury rates during 2013, as well as net outflows from run-off businesses in the Americas, caused the fair  value of the debt securities portfolio to decline from 2012 to 2013. Aegon regularly monitors industry sectors and individual debt securities for indicators of impairment. These indicators may include one  or more of the following: 1) deteriorating market to book ratio, 2) increasing industry risk factors, 3) deteriorating financial condition  of the issuer, 4) covenant violations of the issuer, 5) high probability of bankruptcy of the issuer, or 6) nationally recognized credit  rating agency downgrades. Additionally, for asset-backed securities, cash flow trends and underlying levels of collateral are monitored.  A security is impaired if there is objective evidence that a loss event has occurred after the initial recognition of the asset that has  a negative impact on the estimated future cash flows. A specific security is considered to be impaired when it is determined that not all  amounts due (both principal and interest) will be collected as contractually scheduled. In the sections below a description is provided on the composition of the categories of debt securities and money market investments.  Individual issuers rated below investment grade in any sector, which have unrealized loss positions greater than EUR 25 million, will  be disclosed separately. Furthermore, quality ratings of investment portfolios are based on a hierarchy of S&P, Moody’s, Fitch, Internal  and NAIC. Aegon Americas, Aegon the Netherlands and Aegon UK hold EUR 5,461 million (2012: EUR 6,141 million) of residential mortgage- backed securities (RMBS), of which EUR 4,300 million (2012: EUR 4,979 million) is held by Aegon Americas, EUR 19 million  (2012: EUR 20 million) by Aegon UK and EUR 1,143 million (2012: EUR 1,141 million) by Aegon the Netherlands. Residential  mortgage-backed securities are securitizations of underlying pools of non-commercial mortgages on real estate. The underlying  residential mortgages have varying credit characteristics and are pooled together and sold in tranches. The following table shows the  breakdown of Aegon USA’s RMBS available-for-sale portfolio. Additionally, Aegon USA has investments in RMBS of EUR 60 million  (2012: EUR 97 million), which are classified as fair value through profit or loss. 1  Reported as part of asset backed securities in the table on page 185. Residential mortgage-backed securities Impairment of financial assets AFS RMBS by quality AAA AA A BBB <BBB Total  amortized  cost Total fair  value GSE guaranteed 19 1,307 - - - 1,326 1,357 Prime Jumbo - 7 2 12 212 233 238 Alt-A - 34 8 8 495 546 642 Negative Amortization Floaters - 30 - 34 666 730 830 Reverse Mortgage RMBS - - - 135 116 251 179 Subprime mortgage  1) 89 204 74 80 528 974 1,003 Manufactured housing  1) 6 9 5 13 15 47 48 Other housing  1) 2 - - - - 2 2 At December 31, 2013 116 1,591 89 281 2,031 4,108 4,300 Of which insured - 145 7 - 233 385 372