Insider Risk Management: A Framework Approach to Internal Security (PDF)
It has some interesting data on the risk posed by insiders. Specifically, they look at the difference between risk from malicious attackers and the risk posed by unintentional breaches or well-intentioned employees (the 'Soft' Insider Threat).
Courion points out one of the most interesting data points:
"CXOs also revealed that the greatest financial impact to their organization was caused by risks related to out-of-date or excessive access rights"I was surprised by that. I intuitively know that soft breaches occur far more often than malicious attacks. But, my intuition also tells me that malicious attacks probably cause far more extensive financial harm. The respondents of this survey tell us that inappropriate permissions lead to greater financial harm than malware, internal fraud, deliberate policy violations, and unauthorized access (among others).
You should look directly at the data. It does vary by country. In the U.S. (where the greatest financial losses were reported by respondents), internal fraud edges out excessive rights, but I'm still surprised to see the financial impact of each is almost equal. And keep closer watch on contractors and temporary employees!