In a new book titled Insidious — How Trusted Employees Steal Millions, written by a former bank employee, 72% of Banks have admitted that their employees commi9tted some form of fraud.

Moreover:
Insider fraud: the most common type of fraud banks see.

Source: http://www.darkreading.com/insiderthreat/security/government/showArticle.jhtml?articleID=220301087
http://www.mementosecurity.com/insidious/