When is a person obliged to report a crime?

About ten years ago, as a nurse, I heard a patient planning to do something illegal -- particularly, to lie to an insurance company about the relationship between her injuries and the car accident so that she could keep all the settlement money. At the time, I decided to disengage but not confront or report her. I opted for that due to concerns about patient privacy, the non-violence of the planned crime, and the fact that the insurance company could detect her lie from her medical records. Recently, I've been thinking about the situation. I'm trying to come up with a principle, and I'm getting all muddled. What is my moral responsibility to intervene or report when I know that another person is planning or has done something illegal -- meaning, something that would violate someone's rights? Does my responsibility change if it's a friend (assumed in confidence) or stranger (overheard in public)? Does it matter if the crime has already taken place or is merely in the works? Where is the line regarding severity of the crime? (I'd obviously report if I even heard a stranger plotting murder.) Also, what if you might be harmed if you report, such as in the case of a gang murder? Is there some basic principle that can clarify when a person is obliged to report knowledge of a crime?