Related Party Transations (Table Text Block)	3 Months Ended	6 Months Ended
	Sep. 30, 2020	Sep. 30, 2020
Related Party Revenue [Abstract]
Related Party Revenue	Quarter Ended September 30,     2020   2019     (Unaudited)     (In thousands) U-Haul management fee revenue from Blackwater $ 6,644 $ 6,318 U-Haul management fee revenue from Mercury   1,196   915   $ 7,840 $ 7,233	Six Months Ended September 30,     2020   2019     (Unaudited)     (In thousands) U-Haul management fee revenue from Blackwater $ 12,792 $ 12,567 U-Haul management fee revenue from Mercury   2,395   1,822   $ 15,187 $ 14,389
Related Party costs and expenses	Quarter Ended September 30,     2020   2019     (Unaudited)     (In thousands) U-Haul lease expenses to Blackwater $ 658 $ 658 U-Haul commission expenses to Blackwater   20,533   18,617   $ 21,191 $ 19,275	Six Months Ended September 30,     2020   2019     (Unaudited)     (In thousands) U-Haul lease expenses to Blackwater $ 1,315 $ 1,316 U-Haul commission expenses to Blackwater   35,865   35,819   $ 37,180 $ 37,135
Related party assets		September 30,   March 31,     2020   2020     (Unaudited)         (In thousands) U-Haul receivable from Blackwater $ 28,246 $ 25,293 U-Haul receivable from Mercury   4,838   9,893 Other (a)   (687)   (402)   $ 32,397 $ 34,784 (a)       Timing differences for intercompany balances with insurance subsidiaries resulting from the three-month difference in reporting periods.