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Available-for-sale investments in an unrealized loss position that are not other-than-temporarily impaired: 

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1 Indicates length of time that individual securities have been in a continuous unrealized loss position. 

Corporate Bonds. The unrealized losses on our investments in corporate bonds and asset-backed securities relate to changes in interest rates and credit-related yield spreads since time of purchase. We do not intend to sell the investments and it is not likely that we will be required to sell the investments before recovery of their amortized cost basis. We do not consider these investments to be other-than-temporarily impaired as of December 31, 2018. 

Mortgage-Backed Debt Securities. The unrealized losses on our investments in U.S. governmental agency mortgage-backed securities and commercial mortgage-backed securities relate to changes in interest rates and credit-related yield spreads since time of purchase. We do not intend to sell the investments and it is not likely that we will be required to sell the investments before recovery of their amortized cost basis. We do not consider these investments to be other-than-temporarily impaired as of December 31, 2018. 

The cost basis and fair value of the available-for-sale debt securities at December 31, 2018, by contractual maturity, is shown below. Expected maturities will differ from contractual maturities because borrowers may have the right to prepay and creditors may have the right to call obligations. 

