Consolidating financial information by industry segment (Balance Sheets) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
ASSETS:
Cash and cash equivalents	$ 1,398,840	$ 494,352	$ 631,951	$ 673,701
Reinsurance recoverables and trade receivables, net	224,447	186,672
Inventories, net	104,180	101,083
Prepaid expenses	450,889	562,904
Investments, fixed maturities and marketable equities	2,606,953	2,492,738
Investments, other	447,902	360,373
Deferred policy acquisition costs, net	92,139	103,118
Other assets	44,261	71,956
Right of use Assets - Financing	920,321	1,080,353
Right of use Assets - Operating	97,186	106,631
Related party assets	34,963	34,784
Subtotal assets	6,422,081	5,594,964
Investments in subsidiaries	0	0
Property, plant and equipment, at cost:
Land	1,059,177	1,032,945
Buildings and improvements	4,996,900	4,663,461
Furniture and equipment	778,732	752,363
Property, plant and equipment (gross)	11,017,589	10,556,222
Less: Accumulated depreciation	(2,992,365)	(2,713,162)
Total property, plant and equipment	8,025,224	7,843,060
Total assets	14,447,305	13,438,024	12,994,322
Liabilities:
Accounts payable and accrued expenses	608,765	554,353
Notes, loans and leases payable	4,678,227	4,621,291
Financing lease liability	561,351
Operating lease liability	97,224	106,443
Policy benefits and losses, claims and loss expenses payable	1,013,582	997,647
Liabilities from investment contracts	1,997,936	1,802,217
Other policyholders' funds and liabilities	34,867	10,190
Deferred income	37,005	31,620
Deferred income taxes, net	1,222,544	1,093,543
Related party liabilities	0	0
Total liabilities	9,690,150	9,217,304
Stockholders' equity:
Additional paid-in capital	453,819	453,819
Accumulated other comprehensive loss	85,910	34,652
Retained earnings	4,884,579	4,399,402
Unearned employee stock ownership plan shares	0	0
Total stockholders' equity	4,757,155	4,220,720
Total liabilities and stockholders' equity	14,447,305	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	552,892	511,520
Rental Trucks [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross	3,629,888	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
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	1,250,267	459,078	604,176	643,918
Reinsurance recoverables and trade receivables, net	99,176	60,073
Inventories, net	104,180	101,083
Prepaid expenses	450,889	562,904
Investments, fixed maturities and marketable equities	0	0
Investments, other	20,988	20,988
Deferred policy acquisition costs, net	0	0
Other assets	41,180	69,128
Right of use Assets - Financing	920,321	1,080,353
Right of use Assets - Operating	96,852	106,631
Related party assets	62,120	41,027
Subtotal assets	3,045,973	2,501,265
Investments in subsidiaries	708,441	668,498
Property, plant and equipment, at cost:
Land	1,059,177	1,032,945
Buildings and improvements	4,996,900	4,663,461
Furniture and equipment	778,732	752,363
Property, plant and equipment (gross)	11,017,589	10,556,222
Less: Accumulated depreciation	(2,992,365)	(2,713,162)
Total property, plant and equipment	8,025,224	7,843,060
Total assets	11,779,638	11,012,823
Liabilities:
Accounts payable and accrued expenses	597,902	545,685
Notes, loans and leases payable	4,666,968	4,609,844
Operating lease liability	96,874	106,443
Policy benefits and losses, claims and loss expenses payable	422,473	410,107
Liabilities from investment contracts	0	0
Other policyholders' funds and liabilities	0	0
Deferred income	37,005	31,620
Deferred income taxes, net	1,180,285	1,063,681
Related party liabilities	25,026	24,275
Total liabilities	7,026,533	6,791,655
Stockholders' equity:
Additional paid-in capital	454,029	454,029
Accumulated other comprehensive loss	81,860	35,100
Retained earnings	4,884,369	4,399,192
Unearned employee stock ownership plan shares	0	0
Total stockholders' equity	4,753,105	4,221,168
Total liabilities and stockholders' equity	11,779,638	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	552,892	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	3,629,888	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] | 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	7,046	4,794	5,868	5,757
Reinsurance recoverables and trade receivables, net	86,615	93,995
Inventories, net	0	0
Prepaid expenses	0	0
Investments, fixed maturities and marketable equities	291,509	288,998
Investments, other	103,715	90,145
Deferred policy acquisition costs, net	0	0
Other assets	614	680
Right of use Assets - Financing	0	0
Right of use Assets - Operating	149	0
Related party assets	7,275	7,137
Subtotal assets	496,923	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	496,923	485,749
Liabilities:
Accounts payable and accrued expenses	3,419	5,530
Notes, loans and leases payable	0	0
Operating lease liability	155	0
Policy benefits and losses, claims and loss expenses payable	201,516	210,341
Liabilities from investment contracts	0	0
Other policyholders' funds and liabilities	80	5,751
Deferred income	0	0
Deferred income taxes, net	11,736	8,447
Related party liabilities	3,655	4,616
Total liabilities	220,561	234,685
Stockholders' equity:
Additional paid-in capital	91,120	91,120
Accumulated other comprehensive loss	20,169	12,581
Retained earnings	161,772	144,062
Unearned employee stock ownership plan shares	0	0
Total stockholders' equity	276,362	251,064
Total liabilities and stockholders' equity	496,923	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] | 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	141,527	30,480	21,907	24,026
Reinsurance recoverables and trade receivables, net	38,656	32,604
Inventories, net	0	0
Prepaid expenses	0	0
Investments, fixed maturities and marketable equities	2,315,444	2,203,740
Investments, other	323,199	249,240
Deferred policy acquisition costs, net	92,139	103,118
Other assets	2,467	2,148
Right of use Assets - Financing	0	0
Right of use Assets - Operating	185	0
Related party assets	14,099	18,629
Subtotal assets	2,927,716	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	2,927,716	2,639,959
Liabilities:
Accounts payable and accrued expenses	7,444	3,138
Notes, loans and leases payable	11,259	11,447
Operating lease liability	195	0
Policy benefits and losses, claims and loss expenses payable	389,593	377,199
Liabilities from investment contracts	1,997,936	1,802,217
Other policyholders' funds and liabilities	34,787	4,439
Deferred income	0	0
Deferred income taxes, net	30,523	21,415
Related party liabilities	1,300	2,670
Total liabilities	2,473,037	2,222,525
Stockholders' equity:
Additional paid-in capital	26,271	26,271
Accumulated other comprehensive loss	121,366	78,550
Retained earnings	304,542	310,113
Unearned employee stock ownership plan shares	0	0
Total stockholders' equity	454,679	417,434
Total liabilities and stockholders' equity	2,927,716	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] | 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	(48,531)	(32,009)
Subtotal assets	(48,531)	(32,009)
Investments in subsidiaries	(708,441)	(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	(756,972)	(700,507)
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	(29,981)	(31,561)
Total liabilities	(29,981)	(31,561)
Stockholders' equity:
Additional paid-in capital	(117,601)	(117,601)
Accumulated other comprehensive loss	(137,485)	(91,579)
Retained earnings	(466,104)	(453,965)
Unearned employee stock ownership plan shares	0	0
Total stockholders' equity	(726,991)	(668,946)
Total liabilities and stockholders' equity	(756,972)	(700,507)
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