Is there an idiom for, " Making decision all by yourself without involving or consulting others and expect them to accept it"?

He made the decision unilaterally.

He made the decision by fiat.

I would use the word arrogate here:

arrogate tr. v.
1. To take or claim for oneself without right; appropriate: "That's how my cousin came to don the hand-tailored suits and to arrogate to himself the glamorous responsibility for ushering to their tables big-name customers"
_{TFD Online}

In your case, the person arrogated to himself the decision-making process.

I can't think of an idiom that does all three of these things:

• a decision

• that disregards others' feedback

• with the expectation that they accept it

I have a few idioms that do parts of what you ask. Consider this a partial answer.

1. Make a decision all by yourself without involving or consulting others: he put the cart before the horse (suggesting he mixed up his priorities) or got ahead of himself (suggesting he took a premature action or insufficient information). Neither works by itself for your intention - there's nothing in either one that requires others' feedback to right the cart or make the action mature.

2. For disregarding feedback, there's little better than a tin ear. It is often applied to people who can't appreciate the subtleties of music, but "tin ear" is also applied to people who disregard another's attitude or mood. Other idioms might include turn a blind eye or shut his eyes to, which both involve ignoring or refusing information that may be inconvenient. The disadvantage of all of these is that none of them need pertain to decisionmaking, nor do they restrict the attitude of the people being ignored.

3. If he assumes that you will accept it, he is taking your consent for granted. In other words, he expects your acceptance. This requires incorporating acceptance or consent as the object of the phrase, and of course it doesn't entail anything else you want it to mean by itself.

This is a partial answer because all three of these idioms together might work, but it'd be pretty weird and idiosyncratic, and perhaps better idioms exist. Here's an example of what combining them looks like:

With this latest decision, Jonathan got ahead of himself, turned a blind eye to his coworkers, and took their acceptance for granted.

On second thought, it felt like I needed to specify "decision." If I do that instead of using the first set of idioms, maybe this would also work:

With this latest decision, Jonathan turned a blind eye to his coworkers and took their acceptance for granted.

"Decide unilaterally" is a nice, precise formulation proposed by @TRomano, and that's what I would say most of the time. But you asked for an idiom, so here's a colorful one. This is the response the shocked colleague would make, upon realizing that the arrogant person made an important decision without consulting him:

And what am I, chopped liver?

This means, I'm so unimportant that you wouldn't bother consulting me?

Taking Liberty is a good choice if you are implying about doing something without asking.

I took the liberty and checked the editorial manager of Renie