Determinant of a matrix polynomial

Let $P = \begin{pmatrix} 1 & 2 \ -1 & 4 \end{pmatrix}$ and $Q = P^3 - 2P^2 - 4P + 13 I_2$, where $I_2$ denotes the identity matrix of order 2. Then the determinant of $Q$ is equal to __________ (answer in integer).