Related Party Transactions (Table Text Block)	12 Months Ended
	Mar. 31, 2021
Related Party Revenue [Abstract]
Related Party Revenue	Years Ended March 31,     2021   2020   2019     (In thousands) U-Haul management fee revenue from Blackwater $ 25,512 $ 24,014 $ 23,986 U-Haul management fee revenue from Mercury   6,091   6,392   5,162   $ 31,603 $ 30,406 $ 29,148
Related Party Cost and Expense	Years Ended March 31,     2021   2020   2019     (In thousands) U-Haul lease expenses to Blackwater $ 2,612 $ 2,631 $ 2,678 U-Haul commission expenses to Blackwater   69,212   62,066   61,434   $ 71,824 $ 64,697 $ 64,112
Due from Related Party, Recap of Assets	March 31,     2021   2020     (In thousands) U-Haul receivable from Blackwater $ 27,116 $ 25,293 U-Haul receivable from Mercury   9,632   9,893 Other (a)   (1,353)   (402)   $ 35,395 $ 34,784