Related Party Transations (Table Text Block)	3 Months Ended	6 Months Ended
	Sep. 30, 2021	Sep. 30, 2021
Related Party Revenue [Abstract]
Related Party Revenue	Quarter Ended September 30,     2021   2020     (Unaudited)     (In thousands) U-Haul management fee revenue from Blackwater $ 7,428 $ 6,644 U-Haul management fee revenue from Mercury   1,319   1,196  $ 8,747 $ 7,840	Six Months Ended September 30,     2021   2020     (Unaudited)     (In thousands) U-Haul management fee revenue from Blackwater $ 14,608 $ 12,792 U-Haul management fee revenue from Mercury   2,588   2,395  $ 17,196 $ 15,187
Related Party costs and expenses	Quarter Ended September 30,     2021   2020     (Unaudited)     (In thousands) U-Haul lease expenses to Blackwater $ 611 $ 658 U-Haul commission expenses to Blackwater   26,536   20,533  $ 27,147 $ 21,191	Six Months Ended September 30,     2021   2020     (Unaudited)     (In thousands) U-Haul lease expenses to Blackwater $ 1,237 $ 1,315 U-Haul commission expenses to Blackwater   49,416   35,865  $ 50,653 $ 37,180
Related party assets		September 30,   March 31,     2021   2021     (Unaudited)         (In thousands) U-Haul receivable from Blackwater $ 34,189 $ 27,116 U-Haul receivable from Mercury   4,610   9,632 Other (a)   (26)   (1,353)   $ 38,773 $ 35,395 (a)