Eww, no. Ignore what they've all told you. That's the wrong way entirely, you'll end up with nothing other than a pretty document and a lot of rejection.
Can you program? Create 3D assets? Run a business?
Quite often, there's no such thing as a 'Game Designer', in the essence of it being the only aspect of one's job. Depending on whether you're with the big boys or an indy job, game ideas are created by either guys in office suites and trickled down through producers and directors, or thought up by the guy who's making the game in his part time.
If you have an idea, that's great. Ideas are cheap and worthless. I'm not trying to crush your dream, or anything, but to even have a remote chance with any publisher you'll need:
A comprehensive design document covering nearly every facet of development. At least 100 pages, and the supporting legal documentation for the IP rights. No publisher worth their salt is going to look at a document without some sort of legal backup, otherwise they'd be inundated with claims that they stole ideas.
Abundant concept art, to give a clear idea of the type of game world you're trying to create. Characters, weapons, locations and so forth.
A game demo showing enough of the gameplay to provide both an example of how it plays and a base to build upon.
Finally, a sales and business pitch that'd win over the company.
Sound like a lot? It is. Think, of all the thousands of people who submit ideas, how many actually flesh out? There's the occasional exception, sure, but by and large you've got Buckley's.
Option B. You make it yourself.
Hard, yes. Expensive, you bet. Low chance of success, included in the cover price. Thousands of indy projects are started, maybe a dozen or so in each thousand are actually finished. It's your best chance, though, by far. But ideas, again, won't get you anywhere alone, unless you're extremely lucky. Don't count on that luck. Get yourself either some programming or 3D skills and a partner or two, preferably someone you know and trust, to help you out.
About the only resource you'll need.
Contrary to popular belief, though, you don't need a huge team to make a good game. Six people made Myth 2, only slightly more are working dedicatedly on Stubbs the Zombie. Outsourcing is the key, and having the financial means to back up that outsourcing is even more important.
Good luck.