Encrypting documents through a proxy key
There is the obvious weakness that if Y ever gets compromized, changing the password will not help. The attacker will be able to decrypt all past and future documents. One way this might happen is if the password gets compromized, and the attacker gains access to the encrypted value of Y.

TrueCrypt volumes work in a similar way. When you create a volume, a master key is created which doesn't change throughout the lifetime of the volume, even if you change the password.