| Available-for-sale Securities |
Amortized cost and estimated fair market value of marketable securities classified as available-for-sale (excluding cash equivalents) at March 31, 2022, were as follows: | | | | | | | | | | | | | | | Amortized | | Gross Unrealized | | Estimated Fair | (In thousands) | | Cost | | Gains | | Losses | | Market Value | Investments due in 3 months or less: | | | | | | | | | | | | | Commercial paper | | $ | 5,996 | | $ | — | | $ | — | | $ | 5,996 | Corporate securities | | | 1,953 | | | — | | | (1) | | | 1,952 | Total | | | 7,949 | | | — | | | (1) | | | 7,948 | Investments due in 4-12 months: | | | | | | | | | | | | | Corporate securities | | | 45,329 | | | — | | | (484) | | | 44,845 | Total | | | 45,329 | | | — | | | (484) | | | 44,845 | Investments due in 12 months or greater: | | | | | | | | | | | | | Corporate securities | | | 225,487 | | | — | | | (4,861) | | | 220,626 | Total | | | 225,487 | | | — | | | (4,861) | | | 220,626 | Total marketable securities | | $ | 278,765 | | $ | — | | $ | (5,346) | | $ | 273,419 |
Amortized cost and estimated fair market value of marketable securities classified as available-for-sale (excluding cash equivalents) at December 31, 2021, were as follows: | | | | | | | | | | | | | | | Amortized | | Gross Unrealized | | Estimated Fair | (In thousands) | | Cost | | Gains | | Losses | | Market Value | Investments due in 3 months or less: | | | | | | | | | | | | | Commercial paper | | $ | 89,965 | | $ | — | | $ | — | | $ | 89,965 | Corporate securities | | | 7,285 | | | — | | | (3) | | | 7,282 | Total | | | 97,250 | | | — | | | (3) | | | 97,247 | Investments due in 4-12 months: | | | | | | | | | | | | | Corporate securities | | | 25,054 | | | — | | | (42) | | | 25,012 | Total | | | 25,054 | | | — | | | (42) | | | 25,012 | Investments due in 12 months or greater: | | | | | | | | | | | | | Corporate securities | | | 251,096 | | | 21 | | | (1,141) | | | 249,976 | Total | | | 251,096 | | | 21 | | | (1,141) | | | 249,976 | Total marketable securities | | $ | 373,400 | | $ | 21 | | $ | (1,186) | | $ | 372,235 |
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| Available-for-sale Securities in an Unrealized Loss Position |
The following table summarizes marketable securities classified as available-for-sale (excluding cash equivalents) in a continuous unrealized loss position for which an allowance for credit losses was not recorded at March 31, 2022: | | | | | | | | | | | | | | | | | | | | | Less Than 12 Months | | 12 Months or Longer | | Total | | | Estimated | | Gross | | Estimated | | Gross | | Estimated | | Gross | | | Fair Market | | Unrealized | | Fair Market | | Unrealized | | Fair Market | | Unrealized | (In thousands) | | Value | | Losses | | Value | | Losses | | Value | | Losses | Corporate securities | | $ | 266,473 | | $ | (5,346) | | $ | — | | $ | — | | $ | 266,473 | | $ | (5,346) | Total marketable securities | | $ | 266,473 | | $ | (5,346) | | $ | — | | $ | — | | $ | 266,473 | | $ | (5,346) |
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