Net book value of investments is as follows:

Cost method investments consist of non-marketable equity securities and are accounted for at historical cost. Trading securities are reported at fair value, with gains or losses resulting from changes in fair value recognized currently in earnings. Investments designated as available-for-sale consists of U.S. Treasury, government, agency debt, corporate debt and equity securities and are reported at fair value, with unrealized gains and losses, net of tax, included in accumulated other comprehensive income (loss).

Investments in available-for-sale securities at fair value were as follows:

The company intends to sell the short-term available-for-sale investments within the next 12 months.

The amortized cost and fair value of our short-term available-for-sale investments at April 30, 2017, by contractual years-to-maturity, are as follows:

All of our investments, excluding trading securities, are subject to periodic impairment review. The impairment analysis requires significant judgment to identify events or circumstances that would likely have a significant adverse effect on the future value of the investment. We consider various factors in determining whether an impairment is other-than-temporary, including the severity and duration of the impairment, forecasted recovery, the financial condition and near-term prospects of the investee, and our ability and intent to hold the investment for a period of time sufficient to allow for any anticipated recovery in market value. There was no impairment recognized in the three and six months ended April 30, 2017. There was no impairment recognized in the three months ended April 30, 2016. For the six months ended April 30, 2016, cost method investments with a carrying amount of $2 million were written down to their fair value of zero, resulting in an impairment charge of $2 million, which is included in other income (expense).

As of April 30, 2017, unrealized losses on our short-term investments in available-for-sale securities were insignificant. Unrealized losses related to these investments are due to interest rate fluctuations as opposed to changes in credit quality. There is no other-than-temporary impairment for these investments at April 30, 2017.