getting between groups variance when using (1|x:y) in the formula

Let's say I have a formula like this

my.fit <- glmer(w ~ 1 + y + (1|x) + (1|x:y) + z)

Then the summary() output gives information about variance and std.dev 
for the random term x:y

and ranef(my.fit, postVar = TRUE) gives estimates and standard 
deviations for x and each value of x:y.

Is there a way to get a single measure of the variance of y within x?

I guess I would have got this information naturally if I had used (1 + y 
| x) instead of (1|x:y), but for computational reasons that was not 
possible or at least not convenient.