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Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands | Dec. 31, 2021 | Mar. 31, 2021
--- | --- | ---
Available-for-sale securities, amortized cost: |  |
Amortized cost | $ 2,559,271 | $ 2,405,612
Available-for-sale securities, fair value: |  |
Estimated market value | 2,743,151 | 2,653,539
US Treasury Securities [Member] |  |
Available-for-sale securities, amortized cost: |  |
Due in one year or less | 69,891 | 90,142
Due after one year through five years | 572,812 | 562,442
Due after five years through ten years | 677,891 | 672,733
Due after ten years | 1,075,247 | 905,567
Amortized cost | 2,395,841 | 2,230,884
Available-for-sale securities, fair value: |  |
Due in one year or less | 71,172 | 91,190
Due after one year through five years | 609,255 | 601,818
Due after five years through ten years | 745,523 | 754,536
Due after ten years | 1,143,769 | 1,019,570
Estimated market value | 2,569,719 | 2,467,114
Mortgage-backed securities [Member] |  |
Available-for-sale securities, amortized cost: |  |
Amortized cost | 163,430 | 174,728
Available-for-sale securities, fair value: |  |
Estimated market value | $ 173,432 | $ 186,425

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