In Kottwitz's 1985 Compositio paper,
[Isocrystals with additional structure](http://www.numdam.org/item/CM_1985__56_2_201_0), 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!