From the FullForm you have seen that your expression is essentially Grid@List@List.

To work with the values you need to remove the Grid, probably best by using the Part function.

E.g. to compute the determinant of a random 4x4 matrix placed in a grid with some divider lines:

matrixInGrid = Grid[RandomReal[1, {4, 4}],

Dividers -> {{False, True, {False}}, {True, True, {False}}}];

matrix = matrixInGrid[[1]];

Det@matrix

Hope this helps.