Investments (Table Text Block)	3 Months Ended
	Jun. 30, 2021
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 $ 104,016 $   9,087 $   – $   (818) $   – $   112,285 U.S. government agency mortgage-backed securities   70,219   782   (7)   (762)   –   70,232 Obligations of states and political subdivisions   215,298   18,323   (106)   (969)   –   232,546 Corporate securities   1,998,177   129,442   (1,079)   (18,456)   (1,041)   2,107,043 Mortgage-backed securities   170,569   10,380   (1)   (2)   –   180,946   $ 2,558,279 $   168,014 $   (1,193) $   (21,007) $   (1,041) $   2,703,052   Available-for-sale investments as of March 31, 2021 were as follows:       Amortized   Gross   Gross   Gross   Allowance for Expected Credit Losses   Estimated           (In thousands) U.S. treasury securities and government obligations $ 92,429 $   12,941 $   – $   – $   – $   105,370 U.S. government agency mortgage-backed securities   61,427   911   (1)   (132)   –   62,205 Obligations of states and political subdivisions   230,521   25,249   (59)   (3)   –   255,708 Corporate securities   1,846,507   199,447   (163)   (641)   (1,319)   2,043,831 Mortgage-backed securities   174,728   11,706   (1)   (8)   –   186,425  $ 2,405,612 $  250,254 $ ( 224 )$ ( 784 )$ ( 1,319 )$  2,653,539
Adjusted Cost and Estimated Market Value of Available-for-sale Investments	June 30, 2021   March 31, 2021     Amortized   Estimated   Amortized   Estimated     (Unaudited)         (In thousands) Due in one year or less $ 57,753 $ 58,374 $ 90,142 $ 91,190 Due after one year through five years   590,448   627,370   562,442   601,818 Due after five years through ten years   641,992   694,439   672,733   754,536 Due after ten years   1,097,517   1,141,923   905,567   1,019,570     2,387,710   2,522,106   2,230,884   2,467,114                   Mortgage-backed securities   170,569   180,946   174,728   186,425 Redeemable preferred stocks   –   –   –   –  $ 2,558,279 $ 2,703,052 $ 2,405,612 $ 2,653,539
Available for sale equity investments	June 30, 2021   March 31, 2021     Cost   Market   Cost   Market     (Unaudited)             (In thousands)                   Common stocks $ 9,775 $ 22,673 $ 9,775 $ 20,440 Non-redeemable preferred stocks   28,034   29,408   20,034   21,677  $ 37,809 $ 52,081 $ 29,809 $ 42,117