Idiom refers to a word, phrase or an expression whose collective meaning differs from their individual (words) meaning.
No stone unturned
Meaning: Make every possible effort, try all options.
Ex.: The cops left no stone unturned to catch hold of the crooks.
By hook or by crook
Meaning: By any means, using any way: right or wrong.
Ex.: Harry wanted that contract by hook or by crook.