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Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands | Sep. 30, 2019 | Mar. 31, 2019
--- | --- | ---
Available-for-sale securities, amortized cost: |  |
Amortized cost | $ 2,277,468 | $ 2,228,961
Available-for-sale securities, fair value: |  |
Estimated market value | 2,386,687 | 2,211,229
US Treasury Securities [Member] |  |
Available-for-sale securities, amortized cost: |  |
Due in one year or less | 83,028 | 71,987
Due after one year through five years | 521,038 | 541,195
Due after five years through ten years | 694,348 | 621,031
Due after ten years | 827,418 | 845,052
Amortized cost | 2,125,832 | 2,079,265
Available-for-sale securities, fair value: |  |
Due in one year or less | 83,337 | 71,954
Due after one year through five years | 536,225 | 540,658
Due after five years through ten years | 729,026 | 614,485
Due after ten years | 880,910 | 834,769
Estimated market value | 2,229,498 | 2,061,866
Mortgage-backed securities [Member] |  |
Available-for-sale securities, amortized cost: |  |
Amortized cost | 150,143 | 148,203
Available-for-sale securities, fair value: |  |
Estimated market value | 155,622 | 147,895
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 | $ 1,567 | $ 1,468

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