Consequences
When a party successfully defends criminal charges on a ground of insanity, the consequences vary from jurisdiction to jurisdiction. Usually, the defendant is committed to a mental institution. On the average, a defendant found not guilty by reason of insanity and committed to a mental institution is confined for twice as long as is a defendant who is found guilty and sent to prison. Very few acquitted insanity defendants are given supervised release, and even fewer are released directly following their verdict.
The detention of an insanity acquittee is limited by law. The acquittee must be allowed periodic review in the mental institution, to determine whether continued treatment is necessary. In addition, a hospital facility may not hold an insanity acquittee indefinitely merely because the acquittee has an antisocial personality (Foucha v. Louisiana, 504 U.S. 71, 112 S. Ct. 1780, 118 L. Ed. 2d 437 [1992]).
http://legal-dictionary.thefreedictionary.com/Criminally+insane