According to U.S. accounting rules, the value of an asset is impaired when the sum of estimated future cash flow from that asset is less than the book value of the asset. At this point an impairment loss should be recognized. This is done by taking the difference between the fair market value and the book value and recording this amount as the loss. This basically records the asset as if it were being acquired brand new at it's fair market value recording this as it's new book value. This is a common occurrence for goodwill where a company will purchase another company for more than the value of the net assets of the target company. Under US GAAP, goodwill is tested annually for impairment.
- Albrecht, S., Stice, E., Stice, J., & Swain, M. (2011). Accounting: Concepts and applications (11th ed.). Mason: South-Western, p.396-397