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Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands | Jun. 30, 2019 | Mar. 31, 2019
--- | --- | ---
Available-for-sale securities, amortized cost: |  |
Amortized cost | $ 2,265,268 | $ 2,228,961
Available-for-sale securities, fair value: |  |
Estimated market value | 2,310,239 | 2,211,229
US Treasury Securities [Member] |  |
Available-for-sale securities, amortized cost: |  |
Due in one year or less | 80,244 | 71,987
Due after one year through five years | 553,626 | 541,195
Due after five years through ten years | 646,677 | 621,031
Due after ten years | 836,126 | 845,052
Amortized cost | 2,116,673 | 2,079,265
Available-for-sale securities, fair value: |  |
Due in one year or less | 80,481 | 71,954
Due after one year through five years | 562,599 | 540,658
Due after five years through ten years | 660,403 | 614,485
Due after ten years | 855,403 | 834,769
Estimated market value | 2,158,886 | 2,061,866
Mortgage-backed securities [Member] |  |
Available-for-sale securities, amortized cost: |  |
Amortized cost | 147,102 | 148,203
Available-for-sale securities, fair value: |  |
Estimated market value | 149,803 | 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 | $ 1,550 | $ 1,468

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