Consolidating financial information by industry segment (Balance Sheets) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Mar. 31, 2018
ASSETS:
Cash and cash equivalents	$ 1,194,012		$ 494,352		$ 673,701	$ 759,388
Reinsurance recoverables and trade receivables, net	224,426		186,672
Inventories, net	105,577		101,083
Prepaid expenses	469,144		562,904
Investments, fixed maturities and marketable equities	2,695,656		2,492,738
Investments, other	489,759		360,373
Deferred policy acquisition costs, net	89,749		103,118
Other assets	47,730		71,956
Right of use assets, financing, net	877,038		1,080,353
Right of use assets, operating	92,505		106,631
Related party assets	35,395		34,784
Subtotal assets	6,320,991		5,594,964
Investments in subsidiaries	0		0
Property, plant and equipment, at cost:
Land	1,075,813		1,032,945
Buildings and improvements	5,163,705		4,663,461
Furniture and equipment	786,505		752,363
Property, plant and equipment (gross)	11,413,668		10,556,222
Less: Accumulated depreciation	(3,083,053)		(2,713,162)
Total property, plant and equipment	8,330,615		7,843,060
Total assets	14,651,606		13,438,024		11,891,713
Liabilities:
Accounts payable and accrued expenses	645,575		554,353
Notes, loans and leases payable	4,668,907		4,621,291
Operating lease liabilities	92,510		106,443
Policy benefits and losses, claims and loss expenses payable	997,701		997,647
Liabilities from investment contracts	2,161,530		1,802,217
Other policyholders' funds and liabilities	12,420		10,190
Deferred income	42,592		31,620
Deferred income taxes	1,178,489		1,093,543
Related party liabilities	0		0
Total liabilities	9,799,724		9,217,304
Stockholders' equity:
Additional paid-in capital	453,819		453,819
Accumulated other comprehensive loss	106,857		34,652
Retained earnings	4,958,359		4,399,402
Unearned employee stock ownership plan shares	0		0
Total stockholders' equity	4,851,882		4,220,720
Total liabilities and stockholders' equity	14,651,606		13,438,024
Rental Trailers and Other Rental Equipment [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross	477,921		511,520
Rental Trucks [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross	3,909,724		3,595,933
Series A Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued	0		0
Series B Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued	0		0
Series A Common Stock [Member]
Stockholders' equity:
Common stock, value, issued	0		0
Amerco Common Stock [Member]
Stockholders' equity:
Common stock, value, issued	10,497		10,497
Common Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value	(525,653)		(525,653)
Preferred Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value	(151,997)		(151,997)
Operating Segments [Member] | Moving and Storage Consolidations [Member]
ASSETS:
Cash and cash equivalents	1,010,275		459,078		643,918	702,036
Reinsurance recoverables and trade receivables, net	118,741		60,073
Inventories, net	105,577		101,083
Prepaid expenses	469,144		562,904
Investments, fixed maturities and marketable equities	0		0
Investments, other	20,733		20,988
Deferred policy acquisition costs, net	0		0
Other assets	44,763		69,128
Right of use assets, financing, net	877,038		1,080,353
Right of use assets, operating	92,245		106,631
Related party assets	54,042		41,027
Subtotal assets	2,792,558		2,501,265
Investments in subsidiaries	741,860		668,498
Property, plant and equipment, at cost:
Land	1,075,813		1,032,945
Buildings and improvements	5,163,705		4,663,461
Furniture and equipment	786,505		752,363
Property, plant and equipment (gross)	11,413,668		10,556,222
Less: Accumulated depreciation	(3,083,053)		(2,713,162)
Total property, plant and equipment	8,330,615		7,843,060
Total assets	11,865,033		11,012,823
Liabilities:
Accounts payable and accrued expenses	636,257		545,685
Notes, loans and leases payable	4,657,720		4,609,844
Operating lease liabilities	92,236		106,443
Policy benefits and losses, claims and loss expenses payable	427,073		410,107
Liabilities from investment contracts	0		0
Other policyholders' funds and liabilities	0		0
Deferred income	42,592		31,620
Deferred income taxes	1,136,149		1,063,681
Related party liabilities	25,413		24,275
Total liabilities	7,017,440		6,791,655
Stockholders' equity:
Additional paid-in capital	454,029		454,029
Accumulated other comprehensive loss	102,568		35,100
Retained earnings	4,958,149		4,399,192
Unearned employee stock ownership plan shares	0		0
Total stockholders' equity	4,847,593		4,221,168
Total liabilities and stockholders' equity	11,865,033		11,012,823
Operating Segments [Member] | Moving and Storage Consolidations [Member] | Rental Trailers and Other Rental Equipment [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross	477,921		511,520
Operating Segments [Member] | Moving and Storage Consolidations [Member] | Rental Trucks [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross	3,909,724		3,595,933
Operating Segments [Member] | Moving and Storage Consolidations [Member] | Series A Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued	0		0
Operating Segments [Member] | Moving and Storage Consolidations [Member] | Series B Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued	0		0
Operating Segments [Member] | Moving and Storage Consolidations [Member] | Series A Common Stock [Member]
Stockholders' equity:
Common stock, value, issued	0		0
Operating Segments [Member] | Moving and Storage Consolidations [Member] | Amerco Common Stock [Member]
Stockholders' equity:
Common stock, value, issued	10,497		10,497
Operating Segments [Member] | Moving and Storage Consolidations [Member] | Common Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value	(525,653)		(525,653)
Operating Segments [Member] | Moving and Storage Consolidations [Member] | Preferred Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value	(151,997)		(151,997)
Operating Segments [Member] | Property and Casualty Insurance [Member]
ASSETS:
Cash and cash equivalents	5,658		4,794		5,757	6,639
Reinsurance recoverables and trade receivables, net	67,069		93,995
Inventories, net	0		0
Prepaid expenses	0		0
Investments, fixed maturities and marketable equities	295,753		288,998
Investments, other	90,412		90,145
Deferred policy acquisition costs, net	0		0
Other assets	436		680
Right of use assets, financing, net	0		0
Right of use assets, operating	92		0
Related party assets	6,854		7,137
Subtotal assets	466,274		485,749
Investments in subsidiaries	0		0
Property, plant and equipment, at cost:
Land	0		0
Buildings and improvements	0		0
Furniture and equipment	0		0
Property, plant and equipment (gross)	0		0
Less: Accumulated depreciation	0		0
Total property, plant and equipment	0		0
Total assets	466,274		485,749
Liabilities:
Accounts payable and accrued expenses	2,029		5,530
Notes, loans and leases payable	0		0
Operating lease liabilities	96		0
Policy benefits and losses, claims and loss expenses payable	178,942	$ 178,942	210,341	$ 210,341
Liabilities from investment contracts	0		0
Other policyholders' funds and liabilities	3,698		5,751
Deferred income	0		0
Deferred income taxes	13,046		8,447
Related party liabilities	5,821		4,616
Total liabilities	203,632		234,685
Stockholders' equity:
Additional paid-in capital	91,120		91,120
Accumulated other comprehensive loss	22,546		12,581
Retained earnings	145,675		144,062
Unearned employee stock ownership plan shares	0		0
Total stockholders' equity	262,642		251,064
Total liabilities and stockholders' equity	466,274		485,749
Operating Segments [Member] | Property and Casualty Insurance [Member] | Rental Trailers and Other Rental Equipment [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross	0		0
Operating Segments [Member] | Property and Casualty Insurance [Member] | Rental Trucks [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross	0		0
Operating Segments [Member] | Property and Casualty Insurance [Member] | Series A Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued	0		0
Operating Segments [Member] | Property and Casualty Insurance [Member] | Series B Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued	0		0
Operating Segments [Member] | Property and Casualty Insurance [Member] | Series A Common Stock [Member]
Stockholders' equity:
Common stock, value, issued	0		0
Operating Segments [Member] | Property and Casualty Insurance [Member] | Amerco Common Stock [Member]
Stockholders' equity:
Common stock, value, issued	3,301		3,301
Operating Segments [Member] | Property and Casualty Insurance [Member] | Common Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value	0		0
Operating Segments [Member] | Property and Casualty Insurance [Member] | Preferred Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value	0		0
Operating Segments [Member] | Life Insurance [Member]
ASSETS:
Cash and cash equivalents	178,079		30,480		24,026	50,713
Reinsurance recoverables and trade receivables, net	38,616		32,604
Inventories, net	0		0
Prepaid expenses	0		0
Investments, fixed maturities and marketable equities	2,399,903		2,203,740
Investments, other	378,614		249,240
Deferred policy acquisition costs, net	89,749		103,118
Other assets	2,531		2,148
Right of use assets, financing, net	0		0
Right of use assets, operating	168		0
Related party assets	13,850		18,629
Subtotal assets	3,101,510		2,639,959
Investments in subsidiaries	0		0
Property, plant and equipment, at cost:
Land	0		0
Buildings and improvements	0		0
Furniture and equipment	0		0
Property, plant and equipment (gross)	0		0
Less: Accumulated depreciation	0		0
Total property, plant and equipment	0		0
Total assets	3,101,510		2,639,959
Liabilities:
Accounts payable and accrued expenses	7,289		3,138
Notes, loans and leases payable	11,187		11,447
Operating lease liabilities	178		0
Policy benefits and losses, claims and loss expenses payable	391,686		377,199
Liabilities from investment contracts	2,161,530		1,802,217
Other policyholders' funds and liabilities	8,722		4,439
Deferred income	0		0
Deferred income taxes	29,294		21,415
Related party liabilities	12,406		2,670
Total liabilities	2,622,292		2,222,525
Stockholders' equity:
Additional paid-in capital	26,271		26,271
Accumulated other comprehensive loss	140,817		78,550
Retained earnings	309,630		310,113
Unearned employee stock ownership plan shares	0		0
Total stockholders' equity	479,218		417,434
Total liabilities and stockholders' equity	3,101,510		2,639,959
Operating Segments [Member] | Life Insurance [Member] | Rental Trailers and Other Rental Equipment [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross	0		0
Operating Segments [Member] | Life Insurance [Member] | Rental Trucks [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross	0		0
Operating Segments [Member] | Life Insurance [Member] | Series A Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued	0		0
Operating Segments [Member] | Life Insurance [Member] | Series B Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued	0		0
Operating Segments [Member] | Life Insurance [Member] | Series A Common Stock [Member]
Stockholders' equity:
Common stock, value, issued	0		0
Operating Segments [Member] | Life Insurance [Member] | Amerco Common Stock [Member]
Stockholders' equity:
Common stock, value, issued	2,500		2,500
Operating Segments [Member] | Life Insurance [Member] | Common Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value	0		0
Operating Segments [Member] | Life Insurance [Member] | Preferred Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value	0		0
Eliminations [Member]
ASSETS:
Cash and cash equivalents	0		0		$ 0	$ 0
Reinsurance recoverables and trade receivables, net	0		0
Inventories, net	0		0
Prepaid expenses	0		0
Investments, fixed maturities and marketable equities	0		0
Investments, other	0		0
Deferred policy acquisition costs, net	0		0
Other assets	0		0
Right of use assets, financing, net	0		0
Right of use assets, operating	0		0
Related party assets	(39,351)		(32,009)
Subtotal assets	(39,351)		(32,009)
Investments in subsidiaries	(741,860)		(668,498)
Property, plant and equipment, at cost:
Land	0		0
Buildings and improvements	0		0
Furniture and equipment	0		0
Property, plant and equipment (gross)	0		0
Less: Accumulated depreciation	0		0
Total property, plant and equipment	0		0
Total assets	(781,211)		(700,507)
Liabilities:
Accounts payable and accrued expenses	0		0
Notes, loans and leases payable	0		0
Operating lease liabilities	0		0
Policy benefits and losses, claims and loss expenses payable	0		0
Liabilities from investment contracts	0		0
Other policyholders' funds and liabilities	0		0
Deferred income	0		0
Deferred income taxes	0		0
Related party liabilities	(43,640)		(31,561)
Total liabilities	(43,640)		(31,561)
Stockholders' equity:
Additional paid-in capital	(117,601)		(117,601)
Accumulated other comprehensive loss	(159,074)		(91,579)
Retained earnings	(455,095)		(453,965)
Unearned employee stock ownership plan shares	0		0
Total stockholders' equity	(737,571)		(668,946)
Total liabilities and stockholders' equity	(781,211)		(700,507)
Eliminations [Member] | Rental Trailers and Other Rental Equipment [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross	0		0
Eliminations [Member] | Rental Trucks [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross	0		0
Eliminations [Member] | Series A Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued	0		0
Eliminations [Member] | Series B Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued	0		0
Eliminations [Member] | Series A Common Stock [Member]
Stockholders' equity:
Common stock, value, issued	0		0
Eliminations [Member] | Amerco Common Stock [Member]
Stockholders' equity:
Common stock, value, issued	(5,801)		(5,801)
Eliminations [Member] | Common Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value	0		0
Eliminations [Member] | Preferred Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value	$ 0		$ 0