Investments (Table Text Block)	9 Months Ended	12 Months Ended
	Dec. 31, 2019	Mar. 31, 2019
Investments Debt Equity Securities [Abstract]
Available-for-Sale Investments	Available-for-sale investments as of December 31, 2019 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     (Unaudited)     (In thousands) U.S. treasury securities and government obligations $ 112,784 $ 9,370 $ (3) $ – $ 122,151 U.S. government agency mortgage-backed securities   39,146   887   (2)   –   40,031 Obligations of states and political subdivisions   292,619   23,669   (134)   (45)   316,109 Corporate securities   1,699,751   102,544   (1,133)   (528)   1,800,634 Mortgage-backed securities   153,880   7,602   (1)   (2)   161,479 Redeemable preferred stocks   1,493   82   –   –   1,575   $ 2,299,673 $ 144,154 $ (1,273) $ (575) $ 2,441,979	9   Available-for-sale investments as of March 31, 2019 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 $ 136,010 $ 2,409 $ (2,104) $ (447) $ 135,868 U.S. government agency mortgage-backed securities   31,101   433   (146)   (19)   31,369 Obligations of states and political subdivisions   298,955   8,079   (233)   (905)   305,896 Corporate securities   1,613,199   14,777   (14,257)   (24,986)   1,588,733 Mortgage-backed securities   148,203   880   (285)   (903)   147,895 Redeemable preferred stocks   1,493   20   –   (45)   1,468   $ 2,228,961 $ 26,598 $ (17,025) $ (27,305) $ 2,211,229
Adjusted Cost and Estimated Market Value of Available-for-sale Investments	December 31, 2019   March 31, 2019     Amortized Cost   Estimated Market Value   Amortized Cost   Estimated Market Value     (Unaudited)         (In thousands) Due in one year or less $ 114,557 $ 115,001 $ 71,987 $ 71,954 Due after one year through five years   554,591   572,647   541,195   540,658 Due after five years through ten years   662,534   707,015   621,031   614,485 Due after ten years   812,618   884,262   845,052   834,769     2,144,300   2,278,925   2,079,265   2,061,866                   Mortgage-backed securities   153,880   161,479   148,203   147,895 Redeemable preferred stocks   1,493   1,575   1,493   1,468   $ 2,299,673 $ 2,441,979 $ 2,228,961 $ 2,211,229
Available for sale equity investments	Equity investments of common stock and non-redeemable preferred stock were as follows:     December 31, 2019   March 31, 2019     Amortized Cost   Estimated Market Value   Amortized Cost   Estimated Market Value     (Unaudited)             (In thousands)                   Common stocks $ 9,775 $ 19,988 $ 10,123 $ 17,379 Non-redeemable preferred stocks   7,451   7,501   7,451   6,789   $ 17,226 $ 27,489 $ 17,574 $ 24,168