Few would dispute the idiomaticity of "kicking the bucket", as the non-literal meaning in this multi-word expression is largely conventionalized.
in the sentence "One approach would be to draw the line by reference [...]" the expression "draw the line" could be classified as either metaphorical (because it still evokes the literal senses of its constituents) or idiomatic (as it is a fixed expression with lexicalized figurative sense).