Discover Financial Services (NYSE:DFS) reported its second-quarter financial results on Wednesday. Here's a look at the key figures from the report.  The Details: Discover reported GAAP earnings of $6.06 per share, beating the estimate of $3.07 per share, and quarterly Net Interest Income (NII) of $3.524 billion. Net interest income for the quarter increased by $347 million, or 11% year-over-year, driven by higher average receivables and net interest margin expansion. Net interest margin was 11.17%, up 11 basis points versus the prior year. Card yield was 15.99%, up 85 basis points from the prior year primarily driven by a lower promotional balance mix and lower payment rates, partially offset by higher interest charge-offs. Interest expense as a ...