An idiom is a phrase that has a meaning of its own that cannot be understood from the
meanings of its individual word

Yes. 'Kick the bucket' as an idiom means to die. 'Kick the bucket' in its literal sense means, well....to kick a bucket.

The former is an idiom, the latter is a phrase.

'All at sea' or simply 'at sea' is an idiomatic expression that means to be confused, lost, cannot find your way, don't know where you are or which way to turn. The fact that it consists of more than one word makes it, by simple definition, a phrase.

Let's call it an 'idiomatic phrase', shall we?

Jeez, Taffy, you 'don't let the grass grow' when it comes to 'making a mountain out of a molehill', do you?