Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Mar. 31, 2018
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,125,089	$ 1,847,422
Available-for-sale securities, fair value:
Estimated market value	2,101,739	1,919,860
Us Treasury, Government, and Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	58,311	36,446
Due after one year through five years	503,227	441,223
Due after five years through ten years	648,241	607,895
Due after ten years	794,811	641,411
Amortized cost	2,004,590	1,726,975
Available-for-sale securities, fair value:
Due in one year or less	58,532	36,674
Due after one year through five years	502,343	450,816
Due after five years through ten years	639,629	626,174
Due after ten years	781,774	671,448
Estimated market value	1,982,278	1,785,112
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	119,006	94,106
Available-for-sale securities, fair value:
Estimated market value	117,951	96,025
Redeemable preferred stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	1,493	2,118
Available-for-sale securities, fair value:
Estimated market value	1,510	2,247
Equity Securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost		24,223
Available-for-sale securities, fair value:
Estimated market value		$ 36,476

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