Related Party Transactions (Table Text Block)	12 Months Ended
	Mar. 31, 2020
Related Party Revenue [Abstract]
Related Party Revenue	Years Ended March 31,     2020   2019   2018     (In thousands) U-Haul interest income revenue from Blackwater $ – $ – $ 3,326 U-Haul management fee revenue from Blackwater   24,014   23,986   23,577 U-Haul management fee revenue from Mercury   6,392   5,162   6,025   $ 30,406 $ 29,148 $ 32,928
Related Party Cost and Expense	Years Ended March 31,     2020   2019   2018     (In thousands) U-Haul lease expenses to Blackwater $ 2,631 $ 2,678 $ 2,684 U-Haul commission expenses to Blackwater   62,066   61,434   58,595   $ 64,697 $ 64,112 $ 61,279
Due from Related Party, Recap of Assets	March 31,     2020   2019     (In thousands) U-Haul receivable from Blackwater $ 25,293 $ 25,158 U-Haul receivable from Mercury   9,893   7,234 Other (a)   (402)   (1,503)   $ 34,784 $ 30,889