free rein

Complete freedom to do what one wants or chooses. Can you believe the boss gave me free rein on this project? Finally, I can present a campaign with my own vision! It seems ludicrous that the government would give a megacorporation like them free rein with so much funding. Surely there ought to be some sort of limitation on it, right? Whoa, you don't totally have free rein with this assignment! You still have to get your topic approved by the professor.

(a) free rein

complete freedom of action or expression.

The image here is of loosening grip on the reins of a horse, allowing it to choose its own course and pace, in contrast to the greater control implied by the next idiom.