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Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands | Jun. 30, 2020 | Mar. 31, 2020
--- | --- | ---
Available-for-sale securities, amortized cost: |  |
Amortized cost | $ 2,317,006 | $ 2,334,215
Available-for-sale securities, fair value: |  |
Estimated market value | 2,372,323 | 2,467,613
US Treasury Securities [Member] |  |
Available-for-sale securities, amortized cost: |  |
Due in one year or less | 141,789 | 128,747
Due after one year through five years | 537,400 | 547,821
Due after five years through ten years | 605,305 | 636,036
Due after ten years | 833,420 | 832,334
Amortized cost | 2,117,914 | 2,144,938
Available-for-sale securities, fair value: |  |
Due in one year or less | 141,448 | 129,420
Due after one year through five years | 541,406 | 566,934
Due after five years through ten years | 618,209 | 678,636
Due after ten years | 875,707 | 897,371
Estimated market value | 2,176,770 | 2,272,361
Mortgage-backed securities [Member] |  |
Available-for-sale securities, amortized cost: |  |
Amortized cost | 197,599 | 187,784
Available-for-sale securities, fair value: |  |
Estimated market value | 194,051 | 193,687
Redeemable preferred stocks [Member] |  |
Available-for-sale securities, amortized cost: |  |
Amortized cost | 1,493 | 1,493
Available-for-sale securities, fair value: |  |
Estimated market value | $ 1,502 | $ 1,565

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