calculated on a GAAP basis). The combined ratio reflects only underwriting results and does not include income from investments or finance and other service income. Underwriting profitability is subject to significant fluctuations due to competition, catastrophic events, weather, economic and social conditions, and other factors.
Underwriting, Operating and Related Expenses. Underwriting, operating and related expenses for the year ended December 31, 2015 decreased by $5,084, or 2.3%, to $213,939 from $219,023 for the comparable 2014 period, primarily due to a decrease in commissions expensed to agents. Our GAAP expense ratio for the year ended December 31, 2015 decreased to 29.0% from 30.6% for the comparable 2014 period. The decrease in underwriting, operating and related expenses and the expense ratio is attributable to decreases in contingent commissions and bonus compensation.
As of December 31, 2014, our portfolio of fixed maturity investments was principally comprised of investment grade corporate fixed maturity securities, U.S. government and agency securities, and asset-backed securities. The portion of our non-investment grade portfolio of fixed maturity investments is primarily comprised of variable rate secured and senior bank loans and high yield bonds. We have no exposure to European sovereign debt.
The following table shows our insured investment securities that are backed by financial guarantors including pre-refunded securities as of December 31, 2014. We do not have any direct investment holdings in a financial guarantee insurance company.
The decreases in prior years reserves during the years ended December 31, 2015, 2014 and 2013 resulted from re-estimations of prior year ultimate loss and LAE liabilities. The 2015 decrease is primarily composed of reductions of $18,644, in our retained automobile reserves and $7,964 in our retained homeowners reserves. The 2014 decrease is primarily composed of reductions of $23,272 in our retained automobile reserves and $8,804 in our retained homeowners reserves. The 2013 decrease is primarily composed of reductions of $23,938 in our retained automobile reserves and $4,740 in our retained homeowners reserves..
Service-based restricted stock awarded in the form of unvested shares is recorded at the market value of the Company’s common stock on the grant date and amortized ratably as compensation expense over the requisite service period. Service-based restricted stock awards generally vest over a three-year period and vest 30% on the first and second anniversaries of the grant date and 40% on the third anniversary of the grant date, except for non-executive employees’ restricted stock awards which vest ratably over a five-year service period and independent directors’ stock awards which vest immediately. Our independent directors are subject to stock ownership guidelines, which require them to have a value equal to four times their annual cash retainer.
Other asset-backed securities: overall credit quality, estimates of prepayment speeds based upon historical trends and characteristics of underlying loans, including assessments of the level and variability of collateral, revenue generating agreements, area licenses agreements, product sourcing agreements and equipment and property leases.
There were no transfers between Level 1 and Level 2 during the years ended December 31, 2015 or 2014.