take smth for granted

(from Idioms in Speech)

to assume, accept something as true, or as a fact, or as certain to happen (without reason or proof)

I Although he was a Tory by habit and condition, there were few institutions he took for granted. (I. Murdoch)

She took it for granted that the doctor had really seen her husband. (Th. Dreiser)

I suppose I may take it for granted that this unnatural marriage will take place; in that case there'll be formalities. (J. Galsworthy)

You told me before he went away that you weren't going to get engaged or married or anything till he came back, so naturally I took it for granted there was some definite arrangement between you. (D. Cusack)