Investments (Table Text Block)	9 Months Ended	12 Months Ended
	Dec. 31, 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 $ 118,140 $   9,039 $   – $   (678) $   – $   126,501 U.S. government agency mortgage-backed securities   57,083   370   –   (2,439)   –   55,014 Obligations of states and political subdivisions   191,141   16,734   (80)   (474)   –   207,321 Corporate securities   2,029,477   157,250   (26)   (5,818)   –   2,180,883 Mortgage-backed securities   163,430   10,003   (1)   –   –   173,432  $ 2,559,271 $  193,396 $ ( 107 )$ ( 9,409 )$  – $  2,743,151	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	December 31, 2021   March 31, 2021     Amortized   Estimated   Amortized   Estimated     (Unaudited)         (In thousands) Due in one year or less $ 69,891 $ 71,172 $ 90,142 $ 91,190 Due after one year through five years   572,812   609,255   562,442   601,818 Due after five years through ten years   677,891   745,523   672,733   754,536 Due after ten years   1,075,247   1,143,769   905,567   1,019,570     2,395,841   2,569,719   2,230,884   2,467,114                   Mortgage-backed securities   163,430   173,432   174,728   186,425  $ 2,559,271 $ 2,743,151 $ 2,405,612 $ 2,653,539
Available for sale equity investments	December 31, 2021   March 31, 2021     Cost   Market   Cost   Market     (Unaudited)             (In thousands)                   Common stocks $ 9,775 $ 24,179 $ 9,775 $ 20,440 Non-redeemable preferred stocks   26,054   28,112   20,034   21,677  $ 35,829 $ 52,291 $ 29,809 $ 42,117