You just got a letter that says the other side should not have to pay all of your medical bills because health insurance already covered part of them. That raises the collateral source rule, a rule about whether payments from a separate source can reduce what an injured person may recover from the party who caused the harm.

In general, a collateral source is a payment or benefit that comes from someone other than the defendant, such as private health insurance, Medicare, or certain employment benefits. Under the traditional rule, those outside payments do not automatically lower the defendant's responsibility for the damage they caused. The idea is that a wrongdoer should not get a discount just because the injured person had insurance or another benefit available.

For an injury claim, this can directly affect the value of damages, especially medical expenses and sometimes lost income. It also affects settlement arguments, because insurers for the at-fault party often try to point to bills that were paid by someone else.

Idaho follows a modified approach. Under Idaho Code § 6-1606 (1987), evidence of collateral source payments is generally kept from the jury, but after a verdict the court may reduce the award by certain collateral-source amounts, with important exceptions, including amounts that must be repaid through subrogation and benefits the injured person paid for. That can matter in crash cases, including chain-reaction collisions in heavy smoke or low-visibility highway conditions, because the final recovery may depend on who paid which bills and whether reimbursement rights exist.