Our investments consist of investment-grade marketable debt securities, auction rate preferred securities, and municipal auction rate notes. These investments, some of which have original maturities beyond one year, are classified as short-term based on their highly liquid nature and because these securities represent the investment of cash that is available for current operations. All are classified as available-for-sale and recorded at fair value. The carrying value of available-for-sale securities approximates fair market value due to interest rates that are reset frequently. As of December 31, 2007 and 2006, we had $115.8 million and $124.8 million in available-for-sale securities. Unrealized holding gains and losses are recorded, net of any tax effect, as a separate component of accumulated other comprehensive income. Unrealized gains and losses on available-for-sale securities were not material as of December 31, 2007 and 2006. The gross realized gains and losses on sales of available-for-sale securities were not material for the years ended December 31, 2007, 2006, and 2005.
As of December 31, 2007 we had approximately $50.5 million of auction rate securities classi-fied as available-for-sale securities on our balance sheet. We made the decision during the fourth quarter of 2007 to sell or exit these at the next auction date. As of January 31, 2008, we had no investments in auction rate securities.
The fair value of available-for-sale debt securities at December 31, 2007, by contractual maturity, is shown below (in thousands):
| Cost basis | Estimated fair value |
|||||
| Due in one year or less | $ | 10,201 | $ | 10,360 | ||
| Due after one year through five years | 2,100 | 2,128 | ||||
| Due after five years through ten years | 6,850 | 6,898 | ||||
| Due after 10 years | 95,916 | 96,456 | ||||
| Total | $ | 115,067 | $ | 115,842 | ||
