Ashiok, Nightmare Weaver has the following abilities:
+2: Exile the top three cards of target opponent's library.
-X: Put a creature card with converted mana cost X exiled with Ashiok, Nightmare Weaver onto the battlefield under your control. That creature is a Nightmare in addition to its other types.
-10: Exile all cards from all opponents' hands and graveyards.
When I use the +2 ability, do I exile the cards face up so that my opponent gets to see them, or can I hide that information?