The best replacement for idiom is metaphor.

The best alternatives to idiom are rated as following:
★★★★★  metaphor
★★★★  cliche
★★★  proverb
★★  hyperbole
★★  colloquialism
★  slang
★  simile
★  axiom
★  figure of speech
★  expression
★  idiomatic expression
★  saying
★  phrase
★  adage
  euphemism
  aphorism
  phrasal verb
  pun
  pattern

Your alternative for idiom is not listed? Suggest!
Speaking about idiom, one probably shouldn't forget about bells on.

Other search terms related to idiom and their ratings are:
★★★★★ bells on
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Google Trends

Scale is based on the average worldwide traffic of idiom in all years. Click on graph for more.
idiom
metaphor
cliche