Natural linewidth of hyperfine levels?

The D₂ line of $^{87}mathrm{Rb}$ is the transition between the $5^2mathrm P_{3/2}$ and the $5^2mathrm S_{1/2}$ 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.