This is not an idiom. When you see AS ___ AS ___ you are looking at A Simile. This is comparing two flat things.
Simile.
the sea was still nothing was moving no one was around
A simile.
A metaphor
It is a simile.
As quick as a wink is a simile. ----
simile
simile It is an example of a simile (uses like or as). A simile in itself though is a type of metaphor.
simile
that man
It is a similie as you are comparing (something) to how sly the fox is. A similie uses the words "like or "as".