I was just looking if something like that exists yesterday, but got disappointed. Nice timing.

Check out CryptPad.org, it’s:

• Open Source; so it can’t be taken away from you should the org cease to exist.
• Self-hostable; although you can use or pay to use other people’s instances.
• Collaborative; with real-time editing like Google docs
• Federated; so you can collaborate with others whether you’re using a self-hosted instance or using any other server.
• End-to-end encrypted: The server admins can’t see any of your data.

Only downsides so far is that the UX is a bit slow when first loading documents and there are no mobile apps.