N.Y. PEN. LAW § 25.00 : NY Code - Section 25.00: Defenses; burden of proof

1.  When  a "defense," other than an "affirmative defense," defined by
  statute is raised at a trial, the people have the burden  of  disproving
  such defense beyond a reasonable doubt.
    2.  When  a defense declared by statute to be an "affirmative defense"
  is raised at a trial, the defendant has the burden of establishing  such
  defense by a preponderance of the evidence.