Investments (Table Text Block)	6 Months Ended	12 Months Ended
	Sep. 30, 2021	Mar. 31, 2021
Investments Debt Equity Securities [Abstract]
Available-for-Sale Investments	Amortized   Gross   Gross   Gross   Allowance for Expected Credit Losses   Estimated     (Unaudited)     (In thousands) U.S. treasury securities and government obligations $ 103,378 $   9,917 $   – $   (508) $   – $ 112,787 U.S. government agency mortgage-backed securities   58,978   478   –   (1,890)   –   57,566 Obligations of states and political subdivisions   203,429   19,461   (76)   (283)   –   222,531 Corporate securities   2,048,283   169,072   (90)   (3,615)   (179)   2,213,471 Mortgage-backed securities   172,606   11,284   (1)   (1)   –   183,888  $ 2,586,674 $  210,212 $ ( 167 )$ ( 6,297 )$ ( 179 )$ 2,790,243	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	September 30, 2021   March 31, 2021     Amortized   Estimated   Amortized   Estimated     (Unaudited)         (In thousands) Due in one year or less $ 48,717 $ 49,480 $ 90,142 $ 91,190 Due after one year through five years   575,511   613,941   562,442   601,818 Due after five years through ten years   693,842   768,285   672,733   754,536 Due after ten years   1,095,998   1,174,649   905,567   1,019,570     2,414,068   2,606,355   2,230,884   2,467,114                   Mortgage-backed securities   172,606   183,888   174,728   186,425  $ 2,586,674 $ 2,790,243 $ 2,405,612 $ 2,653,539
Available for sale equity investments	September 30, 2021   March 31, 2021     Cost   Market   Cost   Market     (Unaudited)             (In thousands)                   Common stocks $ 9,775 $ 24,728 $ 9,775 $ 20,440 Non-redeemable preferred stocks   28,034   30,176   20,034   21,677  $ 37,809 $ 54,904 $ 29,809 $ 42,117