Consolidating financial information by industry segment (Balance Sheets) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Mar. 31, 2020
ASSETS:
Cash and cash equivalents	$ 2,430,233	$ 1,194,012	$ 1,215,568	$ 494,352
Reinsurance recoverables and trade receivables, net	227,935	224,426
Inventories, net	142,955	105,577
Prepaid expenses	202,339	469,144
Investments, fixed maturities and marketable equities	2,845,147	2,695,656
Investments, other	571,942	489,759
Deferred policy acquisition costs, net	97,138	89,749
Other assets	51,626	47,730
Right of use Assets - Financing	752,990	877,038
Right of use Assets - Operating	85,237	92,505
Related party assets	38,773	35,395
Subtotal assets	7,446,315	6,320,991
Investments in subsidiaries	0	0
Property, plant and equipment, at cost:
Land	1,172,648	1,075,813
Buildings and improvements	5,477,544	5,163,705
Furniture and equipment	811,337	786,505
Property, plant and equipment (gross)	12,277,075	11,413,668
Less: Accumulated depreciation	(3,386,162)	(3,083,053)
Total property, plant and equipment	8,890,913	8,330,615
Total assets	16,337,228	14,651,606	14,123,478
Liabilities:
Accounts payable and accrued expenses	694,284	645,575
Notes, loans and leases payable	5,335,278	4,668,907
Operating lease liability	85,199	92,510
Policy benefits and losses, claims and loss expenses payable	1,016,505	997,701
Liabilities from investment contracts	2,276,829	2,161,530
Other policyholders' funds and liabilities	10,943	12,420
Deferred income	51,212	42,592
Deferred income taxes, net	1,312,281	1,178,489
Related party liabilities	0	0
Total liabilities	10,782,531	9,799,724
Stockholders' equity:
Additional paid-in capital	453,819	453,819
Accumulated other comprehensive loss	74,207	106,857
Retained earnings	5,693,824	4,958,359
Total stockholders' equity	5,554,697	4,851,882
Total liabilities and stockholders' equity	16,337,228	14,651,606
Rental Trailers and Other Rental Equipment [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross	536,814	477,921
Rental Trucks [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross	4,278,732	3,909,724
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
Serial 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	2,393,762	1,010,275	1,163,253	459,078
Reinsurance recoverables and trade receivables, net	112,319	118,741
Inventories, net	142,955	105,577
Prepaid expenses	202,339	469,144
Investments, fixed maturities and marketable equities	0	0
Investments, other	20,745	20,733
Deferred policy acquisition costs, net	0	0
Other assets	48,190	44,763
Right of use Assets - Financing	752,990	877,038
Right of use Assets - Operating	84,949	92,245
Related party assets	55,175	54,042
Subtotal assets	3,813,424	2,792,558
Investments in subsidiaries	738,116	741,860
Property, plant and equipment, at cost:
Land	1,172,648	1,075,813
Buildings and improvements	5,477,544	5,163,705
Furniture and equipment	811,337	786,505
Property, plant and equipment (gross)	12,277,075	11,413,668
Less: Accumulated depreciation	(3,386,162)	(3,083,053)
Total property, plant and equipment	8,890,913	8,330,615
Total assets	13,442,453	11,865,033
Liabilities:
Accounts payable and accrued expenses	685,563	636,257
Notes, loans and leases payable	5,324,373	4,657,720
Operating lease liability	84,905	92,236
Policy benefits and losses, claims and loss expenses payable	442,160	427,073
Liabilities from investment contracts	0	0
Other policyholders' funds and liabilities	0	0
Deferred income	51,212	42,592
Deferred income taxes, net	1,277,383	1,136,149
Related party liabilities	25,956	25,413
Total liabilities	7,891,552	7,017,440
Stockholders' equity:
Additional paid-in capital	454,029	454,029
Accumulated other comprehensive loss	70,411	102,568
Retained earnings	5,693,614	4,958,149
Total stockholders' equity	5,550,901	4,847,593
Total liabilities and stockholders' equity	13,442,453	11,865,033
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	536,814	477,921
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	4,278,732	3,909,724
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] | Serial 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	7,293	5,658	3,483	4,794
Reinsurance recoverables and trade receivables, net	67,217	67,069
Inventories, net	0	0
Prepaid expenses	0	0
Investments, fixed maturities and marketable equities	292,986	295,753
Investments, other	105,474	90,412
Deferred policy acquisition costs, net	0	0
Other assets	755	436
Right of use Assets - Financing	0	0
Right of use Assets - Operating	155	92
Related party assets	8,294	6,854
Subtotal assets	482,174	466,274
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	482,174	466,274
Liabilities:
Accounts payable and accrued expenses	3,575	2,029
Notes, loans and leases payable	0	0
Operating lease liability	152	96
Policy benefits and losses, claims and loss expenses payable	179,116	178,942
Liabilities from investment contracts	0	0
Other policyholders' funds and liabilities	3,476	3,698
Deferred income	0	0
Deferred income taxes, net	13,152	13,046
Related party liabilities	3,600	5,821
Total liabilities	203,071	203,632
Stockholders' equity:
Additional paid-in capital	91,120	91,120
Accumulated other comprehensive loss	19,692	22,546
Retained earnings	164,990	145,675
Total stockholders' equity	279,103	262,642
Total liabilities and stockholders' equity	482,174	466,274
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] | Serial 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	29,178	178,079	48,832	30,480
Reinsurance recoverables and trade receivables, net	48,399	38,616
Inventories, net	0	0
Prepaid expenses	0	0
Investments, fixed maturities and marketable equities	2,552,161	2,399,903
Investments, other	445,723	378,614
Deferred policy acquisition costs, net	97,138	89,749
Other assets	2,681	2,531
Right of use Assets - Financing	0	0
Right of use Assets - Operating	133	168
Related party assets	14,325	13,850
Subtotal assets	3,189,738	3,101,510
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	3,189,738	3,101,510
Liabilities:
Accounts payable and accrued expenses	5,146	7,289
Notes, loans and leases payable	10,905	11,187
Operating lease liability	142	178
Policy benefits and losses, claims and loss expenses payable	395,229	391,686
Liabilities from investment contracts	2,276,829	2,161,530
Other policyholders' funds and liabilities	7,467	8,722
Deferred income	0	0
Deferred income taxes, net	21,746	29,294
Related party liabilities	13,261	12,406
Total liabilities	2,730,725	2,622,292
Stockholders' equity:
Additional paid-in capital	26,271	26,271
Accumulated other comprehensive loss	112,519	140,817
Retained earnings	317,723	309,630
Total stockholders' equity	459,013	479,218
Total liabilities and stockholders' equity	3,189,738	3,101,510
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] | Serial 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
Consolidation, 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	0	0
Right of use Assets - Operating	0	0
Related party assets	(39,021)	(39,351)
Subtotal assets	(39,021)	(39,351)
Investments in subsidiaries	(738,116)	(741,860)
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	(777,137)	(781,211)
Liabilities:
Accounts payable and accrued expenses	0	0
Notes, loans and leases payable	0	0
Operating lease liability	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, net	0	0
Related party liabilities	(42,817)	(43,640)
Total liabilities	(42,817)	(43,640)
Stockholders' equity:
Additional paid-in capital	(117,601)	(117,601)
Accumulated other comprehensive loss	(128,415)	(159,074)
Retained earnings	(482,503)	(455,095)
Total stockholders' equity	(734,320)	(737,571)
Total liabilities and stockholders' equity	(777,137)	(781,211)
Consolidation, 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
Consolidation, Eliminations [Member] | Rental Trucks [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross	0	0
Consolidation, Eliminations [Member] | Series A Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued	0	0
Consolidation, Eliminations [Member] | Series B Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued	0	0
Consolidation, Eliminations [Member] | Serial Common Stock [Member]
Stockholders' equity:
Common stock, value, issued	0	0
Consolidation, Eliminations [Member] | Amerco Common Stock [Member]
Stockholders' equity:
Common stock, value, issued	(5,801)	(5,801)
Consolidation, Eliminations [Member] | Common Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value	0	0
Consolidation, Eliminations [Member] | Preferred Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value	$ 0	$ 0