Investments (Table Text Block)	6 Months Ended
	Sep. 30, 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 $ 92,389 $ 13,609 $ - $ - $ - $ 105,998 U.S. government agency mortgage-backed securities   110,514   2,808   (1)   -   -   113,321 Obligations of states and political subdivisions   257,153   26,803   (79)   -   -   283,877 Corporate securities   1,602,921   151,789   (912)   (5,140)   (4,258)   1,744,400 Mortgage-backed securities   198,829   11,333   (1)   (128)   -   210,033 Redeemable preferred stocks   1,493   37   -   -   -   1,530   $ 2,263,299 $ 206,379 $ (993) $ (5,268) $ (4,258) $ 2,459,159   Available-for-sale investments at March 31, 2020 were as follows:     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	September 30, 2020   March 31, 2020     Amortized Cost   Estimated Market Value   Amortized Cost   Estimated Market Value     (Unaudited)         (In thousands) Due in one year or less $ 112,348 $ 110,836 $ 128,747 $ 129,420 Due after one year through five years   532,931   562,558   547,821   566,934 Due after five years through ten years   626,777   691,999   636,036   678,636 Due after ten years   790,921   882,203   832,334   897,371     2,062,977   2,247,596   2,144,938   2,272,361                   Mortgage-backed securities   198,829   210,033   187,784   193,687 Redeemable preferred stocks   1,493   1,530   1,493   1,565   $ 2,263,299 $ 2,459,159 $ 2,334,215 $ 2,467,613
Available for sale equity investments	September 30, 2020   March 31, 2020     Amortized Cost   Estimated Market Value   Amortized Cost   Estimated Market Value     (Unaudited)             (In thousands)                   Common stocks $ 9,775 $ 17,927 $ 9,775 $ 20,015 Non-redeemable preferred stocks   18,187   18,256   5,076   5,110   $ 27,962 $ 36,183 $ 14,851 $ 25,125