Standard & Poor's Rating Services upgraded its credit rating for Philadelphia's general-obligation bond to A+, Mayor Nutter's administration announced today.

S&P has been steadily increasing the city's rating since mid-2012, when Philly's bonds were BBB. The highest possible grade is AAA+.

It's great news for Philly taxpayers because stronger credit rating means lower borrowing costs. But Philly still lags behind other large U.S. cities.

Here's S&P's definition of an "A" rating: "Strong capacity to meet financial commitments, but somewhat susceptible to adverse economic conditions and changes in circumstances."

The agency attaches es a "+" or "-" to reflect "relative standing within the major rating categories."