In Kottwitz's 1985 Compositio paper, Isocrystals with additional structure, first page, paragraph 4:

Let $\mathbb{D}$ be the diagonalizable pro-algebraic group over $\mathbb{Q}_p$ with character group $\mathbb{Q}$

What is this $\mathbb{D}$?... I completely have no idea (and it is used repeatedly in the paper)... Where can I find a more detailed definition?

Thank you!