[Idiom] In future or in the future (why is it an idiom)?

[Nonverbis]

https://www.oxfordlearnersdictionaries.com/definition/english/future_1?q=future

• 1. in the future The movie is set in the future.

Idiom

• in future
  (North American English in the future)
  
  ​from now on
  
  Please be more careful in future.
  In future, make sure the door is never left unlocked.
  
  As we can see from the article, in the future is an ordinary use of the word.
  But in future is an idiom.
  
  A definition of an idiom: https://www.oxfordlearnersdictionaries.com/definition/english/idiom?q=idiom
  
  To my mind the meaninig of point 1 and the idiom is the same.
  So, why is it an idiom?
  
  

 

[probus]

It is an idiom only in accordance with the second meaning given by Oxford Learners. To put it another way, the words have their ordinary meaning, but apparently it is used in AmE but not in BrE.

 

[jutfrank]

The phrases in the box marked 'idioms' are not idioms in the sense that the word is normally used in English teaching (as in sense 1 of your second link). They are just phrases/expressions.