Investments (Table Text Block)	3 Months Ended	12 Months Ended
	Jun. 30, 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 $ 84,814 $ 12,192 $ - $ - $ - $ 97,006 U.S. government agency mortgage-backed securities   126,599   3,458   (1)   (39)   -   130,017 Obligations of states and political subdivisions   270,476   21,364   (141)   (2)   -   291,697 Corporate securities   1,636,025   66,359   (1,759)   (37,168)   (5,407)   1,658,050 Mortgage-backed securities   197,599   2,485   (2)   (6,031)   -   194,051 Redeemable preferred stocks   1,493   14   -   (5)   -   1,502   $ 2,317,006 $ 105,872 $ (1,903) $ (43,245) $ (5,407) $ 2,372,323	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	June 30, 2020   March 31, 2020     Amortized Cost   Estimated Market Value   Amortized Cost   Estimated Market Value     (Unaudited)         (In thousands) Due in one year or less $ 141,789 $ 141,448 $ 128,747 $ 129,420 Due after one year through five years   537,400   541,406   547,821   566,934 Due after five years through ten years   605,305   618,209   636,036   678,636 Due after ten years   833,420   875,707   832,334   897,371     2,117,914   2,176,770   2,144,938   2,272,361                   Mortgage-backed securities   197,599   194,051   187,784   193,687 Redeemable preferred stocks   1,493   1,502   1,493   1,565   $ 2,317,006 $ 2,372,323 $ 2,334,215 $ 2,467,613
Available for sale equity investments	June 30, 2020   March 31, 2020     Amortized Cost   Estimated Market Value   Amortized Cost   Estimated Market Value     (Unaudited)             (In thousands)                   Common stocks $ 9,775 $ 16,595 $ 9,775 $ 20,015 Non-redeemable preferred stocks   5,076   4,604   5,076   5,110   $ 14,851 $ 21,199 $ 14,851 $ 25,125