What: Shares of Sallie Mae (NASDAQ:SLM)are popping today as the company beat Wall Street's fourth-quarter earnings expectations. Shares are trading higher by about 11% as of 11 a.m. ET. 

So what: Earnings for the fourth quarter came in at $90 million, or $0.20 per diluted share, substantially higher than the year-ago haul of $15 million, or $0.03 per diluted share. The consensus estimate called for earnings of $0.18 per diluted share.

Sallie Mae noted that the increase was largely the result of a $37 million increase in net interest income, a $58 million increase in gains on sale, and a $4 million decrease in expenses.

Impressively, the company's net interest margin, or the spread between what it earns on its loans and its funding costs, jumped 47 basis points compared to the year-ago period to 5.48%.

Now what: Sallie Mae guided for a lighter 2016, indicating that it expected diluted "core" earnings per share of $0.49 to $0.51 for the full year on private loan originations of $4.6 billion. The company's "core" earnings measure excludes unrealized gains or losses from derivative contracts.

Since 2014, the full-year impact of adjustments to get to "core" earnings have been relatively modest -- roughly $1 million up and down in each of the last two years. For this reason, the guidance for core earnings is generally a good benchmark for its GAAP results.