Property of the norm of an ideal

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In Stewart/Tall's book on ANT they authors claim that if $ 0 \ne \mathfrak{a}$ is an ideal of $\mathfrak{O}_K$ the ring of integers of a number field $K$, then $N(\mathfrak{a}) \in \mathfrak{a}$. The proof they give is unclear to me, can someone please explain this?