. u.s . treasury and government agencies , agency residential mortgage-backed and agency commercial mortgage-backed securities collectively represented 59% ( 59 % ) of the investment securities portfolio at december 31 , 2012 . at december 31 , 2012 , the securities available for sale portfolio included a net unrealized gain of $ 1.6 billion , which represented the difference between fair value and amortized cost . the comparable amount at december 31 , 2011 was a net unrealized loss of $ 41 million