Investments (Table Text Block)	9 Months Ended	12 Months Ended
	Dec. 31, 2020	Mar. 31, 2020
Investments Debt Equity Securities [Abstract]
Available-for-Sale Investments	Amortized Cost   Gross Unrealized Gains   Gross Unrealized Losses More than 12 Months   Gross Unrealized Losses Less than 12 Months   Allowance for Expected Credit Losses   Estimated Market Value     (Unaudited)     (In thousands) U.S. treasury securities and government obligations $ 92,300 $ 13,787 $ - $ - $ - $ 106,087 U.S. government agency mortgage-backed securities   88,190   1,208   (1)   (53)   -   89,344 Obligations of states and political subdivisions   241,299   25,533   (63)   (26)   -   266,743 Corporate securities   1,725,211   169,856   (613)   (5,459)   (2,315)   1,886,680 Mortgage-backed securities   202,753   13,896   (1)   (209)   -   216,439 Redeemable preferred stocks   1,493   27   -   -   -   1,520   $ 2,351,246 $ 224,307 $ (678) $ (5,747) $ - $ 2,566,813	Amortized Cost   Gross Unrealized Gains   Gross Unrealized Losses More than 12 Months   Gross Unrealized Losses Less than 12 Months   Estimated Market Value           (In thousands) U.S. treasury securities and government obligations $ 112,421 $ 7,959 $ (1) $ - $ 120,379 U.S. government agency mortgage-backed securities   88,449   759   (1)   (373)   88,834 Obligations of states and political subdivisions   287,643   20,664   (155)   -   308,152 Corporate securities   1,656,425   100,302   (919)   (812)   1,754,996 Mortgage-backed securities   187,784   6,011   (1)   (107)   193,687 Redeemable preferred stocks   1,493   72   -   -   1,565   $ 2,334,215 $ 135,767 $ (1,077) $ (1,292) $ 2,467,613
Adjusted Cost and Estimated Market Value of Available-for-sale Investments	December 31, 2020   March 31, 2020     Amortized Cost   Estimated Market Value   Amortized Cost   Estimated Market Value     (Unaudited)         (In thousands) Due in one year or less $ 113,929 $ 114,681 $ 128,747 $ 129,420 Due after one year through five years   555,675   590,071   547,821   566,934 Due after five years through ten years   656,915   726,982   636,036   678,636 Due after ten years   820,481   917,120   832,334   897,371     2,147,000   2,348,854   2,144,938   2,272,361                   Mortgage-backed securities   202,753   216,439   187,784   193,687 Redeemable preferred stocks   1,493   1,520   1,493   1,565   $ 2,351,246 $ 2,566,813 $ 2,334,215 $ 2,467,613
Available for sale equity investments	December 31, 2020   March 31, 2020     Amortized Cost   Estimated Market Value   Amortized Cost   Estimated Market Value     (Unaudited)             (In thousands)                   Common stocks $ 9,775 $ 19,445 $ 9,775 $ 20,015 Non-redeemable preferred stocks   20,034   20,695   5,076   5,110   $ 29,809 $ 40,140 $ 14,851 $ 25,125