Consolidating financial information by industry segment (Balance Sheets) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019
ASSETS:
Cash and cash equivalents	$ 1,215,568	$ 494,352	$ 525,231	$ 673,701
Reinsurance recoverables and trade receivables, net	213,397	186,672
Inventories, net	100,595	101,083
Prepaid expenses	447,073	562,904
Investments, fixed maturities and marketable equities	2,495,342	2,492,738
Investments, other	425,527	360,373
Deferred policy acquisition costs, net	93,407	103,118
Other assets	71,314	71,956
Right of use Assets - Financing	970,361	1,080,353
Right of use Assets - Operating	101,946	106,631
Related party assets	32,397	34,784
Subtotal assets	6,166,927	5,594,964
Investments in subsidiaries	0	0
Property, plant and equipment, at cost:
Land	1,052,205	1,032,945
Buildings and improvements	4,863,590	4,663,461
Furniture and equipment	767,290	752,363
Property, plant and equipment (gross)	10,859,224	10,556,222
Less: Accumulated depreciation	(2,902,673)	(2,713,162)
Total property, plant and equipment	7,956,551	7,843,060
Total assets	14,123,478	13,438,024	12,856,202
Liabilities:
Accounts payable and accrued expenses	630,666	554,353
Notes, loans and leases payable	4,701,225	4,621,291
Operating lease liability	101,976	106,443
Policy benefits and losses, claims and loss expenses payable	1,015,189	997,647
Liabilities from investment contracts	1,838,280	1,802,217
Other policyholders' funds and liabilities	5,426	10,190
Deferred income	41,715	31,620
Deferred income taxes, net	1,183,370	1,093,543
Related party liabilities	0	0
Total liabilities	9,517,847	9,217,304
Stockholders' equity:
Additional paid-in capital	453,819	453,819
Accumulated other comprehensive loss	78,156	34,652
Retained earnings	4,740,809	4,399,402
Unearned employee stock ownership plan shares	0	0
Total stockholders' equity	4,605,631	4,220,720
Total liabilities and stockholders' equity	14,123,478	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	531,465	511,520
Rental Trucks [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross	3,644,674	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	1,163,253	459,078	498,984	643,918
Reinsurance recoverables and trade receivables, net	78,982	60,073
Inventories, net	100,595	101,083
Prepaid expenses	447,073	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	68,169	69,128
Right of use Assets - Financing	970,361	1,080,353
Right of use Assets - Operating	101,482	106,631
Related party assets	39,481	41,027
Subtotal assets	2,990,384	2,501,265
Investments in subsidiaries	705,467	668,498
Property, plant and equipment, at cost:
Land	1,052,205	1,032,945
Buildings and improvements	4,863,590	4,663,461
Furniture and equipment	767,290	752,363
Property, plant and equipment (gross)	10,859,224	10,556,222
Less: Accumulated depreciation	(2,902,673)	(2,713,162)
Total property, plant and equipment	7,956,551	7,843,060
Total assets	11,652,402	11,012,823
Liabilities:
Accounts payable and accrued expenses	622,424	545,685
Notes, loans and leases payable	4,689,896	4,609,844
Operating lease liability	101,493	106,443
Policy benefits and losses, claims and loss expenses payable	425,376	410,107
Liabilities from investment contracts	0	0
Other policyholders' funds and liabilities	0	0
Deferred income	41,715	31,620
Deferred income taxes, net	1,143,886	1,063,681
Related party liabilities	25,928	24,275
Total liabilities	7,050,718	6,791,655
Stockholders' equity:
Additional paid-in capital	454,029	454,029
Accumulated other comprehensive loss	74,209	35,100
Retained earnings	4,740,599	4,399,192
Unearned employee stock ownership plan shares	0	0
Total stockholders' equity	4,601,684	4,221,168
Total liabilities and stockholders' equity	11,652,402	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	531,465	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	3,644,674	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	3,483	4,794	11,576	5,757
Reinsurance recoverables and trade receivables, net	91,371	93,995
Inventories, net	0	0
Prepaid expenses	0	0
Investments, fixed maturities and marketable equities	285,987	288,998
Investments, other	99,542	90,145
Deferred policy acquisition costs, net	0	0
Other assets	816	680
Right of use Assets - Financing	0	0
Right of use Assets - Operating	262	0
Related party assets	6,281	7,137
Subtotal assets	487,742	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	487,742	485,749
Liabilities:
Accounts payable and accrued expenses	2,890	5,530
Notes, loans and leases payable	0	0
Operating lease liability	271	0
Policy benefits and losses, claims and loss expenses payable	204,592	210,341
Liabilities from investment contracts	0	0
Other policyholders' funds and liabilities	658	5,751
Deferred income	0	0
Deferred income taxes, net	10,491	8,447
Related party liabilities	4,490	4,616
Total liabilities	223,392	234,685
Stockholders' equity:
Additional paid-in capital	91,120	91,120
Accumulated other comprehensive loss	17,243	12,581
Retained earnings	152,686	144,062
Unearned employee stock ownership plan shares	0	0
Total stockholders' equity	264,350	251,064
Total liabilities and stockholders' equity	487,742	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	48,832	30,480	14,671	24,026
Reinsurance recoverables and trade receivables, net	43,044	32,604
Inventories, net	0	0
Prepaid expenses	0	0
Investments, fixed maturities and marketable equities	2,209,355	2,203,740
Investments, other	304,997	249,240
Deferred policy acquisition costs, net	93,407	103,118
Other assets	2,329	2,148
Right of use Assets - Financing	0	0
Right of use Assets - Operating	202	0
Related party assets	14,213	18,629
Subtotal assets	2,716,379	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	2,716,379	2,639,959
Liabilities:
Accounts payable and accrued expenses	5,352	3,138
Notes, loans and leases payable	11,329	11,447
Operating lease liability	212	0
Policy benefits and losses, claims and loss expenses payable	385,221	377,199
Liabilities from investment contracts	1,838,280	1,802,217
Other policyholders' funds and liabilities	4,768	4,439
Deferred income	0	0
Deferred income taxes, net	28,993	21,415
Related party liabilities	1,107	2,670
Total liabilities	2,275,262	2,222,525
Stockholders' equity:
Additional paid-in capital	26,271	26,271
Accumulated other comprehensive loss	114,835	78,550
Retained earnings	297,511	310,113
Unearned employee stock ownership plan shares	0	0
Total stockholders' equity	441,117	417,434
Total liabilities and stockholders' equity	2,716,379	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	(27,578)	(32,009)
Subtotal assets	(27,578)	(32,009)
Investments in subsidiaries	(705,467)	(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	(733,045)	(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	(31,525)	(31,561)
Total liabilities	(31,525)	(31,561)
Stockholders' equity:
Additional paid-in capital	(117,601)	(117,601)
Accumulated other comprehensive loss	(128,131)	(91,579)
Retained earnings	(449,987)	(453,965)
Unearned employee stock ownership plan shares	0	0
Total stockholders' equity	(701,520)	(668,946)
Total liabilities and stockholders' equity	(733,045)	(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