How do you realize a big idea?

This thought has been holding coup in my mind of late. You can probably surmise the reason - I've had a big idea. It's probably my biggest yet, and I'm kindof an idea guy, so that's saying something. Like, it could be a genre-creating idea. I'm really, really excited about it, but then I fall into depression as soon as this thought appears in my conscious: "Ok, so now what?"

Inevitably, this is where I stop when I get most of my ideas. My ideas are usually big ideas, which is not to necessarily say they are good ideas, just that they take a lot of effort to accomplish - far more than one person could accomplish in any reasonable time-period.

As an example, this idea revolves around creating a computer-based game with an online component. And it's a big thing - the type of endeavor that will require a competent marketing team, deep-level engine development, high-level story-telling and model development, as well as an administration and moderation team - and possibly a decent legal team as well. In all - a lot of people with very different talents coming together under one vision. But I'm a web developer, not a game developer. I can express to you with perfect precision how many contacts I have in the game development industry. It's zero.

So I ask you, how does one take an idea and make it happen?