In Kottwitz's 1985 Compositio paper,
Isocrystals with additional structure
www.numdam.org/item/CM_1985__56_2_201_0

first page, paragraph 4:
Let D be the diagonalizable pro-algebraic group over Q_p with character group Q

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

Thank you!