How can I get rid of the white background behind the document?

Go to Window > Color > Double-click on the Fill box > Choose your desired color.

It's the pasteboard. The W key toggles its visibility.

The white background behind the page represents the paper. It does not print. You can change the paper color by modifying the [Paper] swatch in the Swatches panel. 

The white background outside the page is the pasteboard. You can use it to hold elements that you're not ready to add to the page yet. They will be stored with the file, but they will not print. If you are trying to make the pasteboard fade back, enable View > Match Pasteboard to Theme Color. 

~Barb 

Why would you need a transparent background? Tell us what you're trying to do.

InDesign documents intended for either print or online use will never have a transparent background.