Answer: The investment is written down to fair value, and only the credit loss component of the impairment loss is recognized in net income.
Explanation: The fair value of the debt is simply its value if you adjust the price of the debt so that a buyer would be earning the market rate of interest. If the fair value of a debt investment that is classified as an available-for-sale investment declines for a reason that is viewed as "other than temporary" because the company has incurred a credit loss on the investment then the investment is written down to fair value, and only the credit loss component of the impairment loss is recognized in net income.