Your guess is as good as mine
Definition 1 of 1
Pro Tip 1/3Uncertainty Expression
Use it when both you and the listener lack knowledge about the subject. 
As for the new policies, your guess is as good as mine.
Pro Tip 2/3Colloquial Use
It's casual language, not typically used in formal writing or speeches. 
Where did she go? Your guess is as good as mine!
Pro Tip 3/3Avoid Overuse
Don't rely on this phrase too much; it can imply a lack of effort or concern. 
I use 'your guess is as good as mine' only when I truly have no insight.