* I don't know. This is the most common translation.
* I don't care. This is often implied, especially if said with a dismissive tone.
* I have no idea. Similar to "I don't know," but can suggest a more profound lack of knowledge.
* It's beyond me. This implies that the issue is too difficult or complicated to understand.
The best translation will always depend on the situation in which "iwan ko" is used.