To be sure you are doing the right thing you should talk to a lawyer.
Maybe the big company will be willing to sign a NDA. That is still not 100% protection though.

Most investors are not so much interested in an idea alone, but more into how you are going to proceed to build, promote, brand it.

Will the company just profit from your profits, will they eventually own the concept?
Do you know how much they want to know before funding you?
Is it possible for you to fund it yourself first, and get funded by the company later on?

There are a lot of variables to consider in this scenario.