Biased response: as a native American English speaker I can assure you that “a million to one” is idiomatically correct.
I suspect that the more complete non-idiomatic version would be “a million to one against” and that the “against” is implicit because the idiom is highly established as expressing a very low probability.
Biased response: as a native American English speaker I can assure you that “a million to one” is idiomatically correct.
I suspect that the more complete non-idiomatic version would be “a million to one against” and that the “against” is implicit because the idiom is highly established as expressing a very low probability.