atomic physics - Natural linewidth of hyperfine levels?

The D2 line of $^{87}\mathrm{Rb}$ is the transition between the $5^2\mathrm P_{3/2}$ and the $5^2\mathrm S_{1/2}$ states. Its linewidth $\Gamma$ is always quoted to be $\sim 6\ \mathrm{MHz}$.

But both states have hyperfine splittings.

The transition now is between each (allowed) pair of hyperfine states: how do we calculate the linewidth of each of these transitions?

data sheet for rubidium-87 here