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Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands | Jun. 30, 2021 | Mar. 31, 2021
--- | --- | ---
Available-for-sale securities, amortized cost: |  |
Amortized cost | $ 2,558,279 | $ 2,405,612
Available-for-sale securities, fair value: |  |
Estimated market value | 2,703,052 | 2,653,539
US Treasury Securities [Member] |  |
Available-for-sale securities, amortized cost: |  |
Due in one year or less | 57,753 | 90,142
Due after one year through five years | 590,448 | 562,442
Due after five years through ten years | 641,992 | 672,733
Due after ten years | 1,097,517 | 905,567
Amortized cost | 2,387,710 | 2,230,884
Available-for-sale securities, fair value: |  |
Due in one year or less | 58,374 | 91,190
Due after one year through five years | 627,370 | 601,818
Due after five years through ten years | 694,439 | 754,536
Due after ten years | 1,141,923 | 1,019,570
Estimated market value | 2,522,106 | 2,467,114
Mortgage-backed securities [Member] |  |
Available-for-sale securities, amortized cost: |  |
Amortized cost | 170,569 | 174,728
Available-for-sale securities, fair value: |  |
Estimated market value | 180,946 | 186,425
Redeemable preferred stocks [Member] |  |
Available-for-sale securities, amortized cost: |  |
Amortized cost | 0 | 0
Available-for-sale securities, fair value: |  |
Estimated market value | $ 0 | $ 0

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