Consolidating financial information by industry segment (Balance Sheets) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
ASSETS:
Cash and cash equivalents	$ 631,951	$ 673,701	$ 984,385	$ 759,388
Reinsurance recoverables and trade receivables, net	200,164	224,785
Inventories, net	103,003	103,504
Prepaid expenses	188,780	174,100
Investments, fixed maturities and marketable equities	2,469,468	2,235,397
Investments, other	336,727	300,736
Deferred policy acquisition costs, net	106,354	136,276
Right of use Assets - Financing	71,821	78,354
Right of use Assets - Operating	1,130,473	0
Other assets	107,974
Related party assets	47,988	30,889
Subtotal assets	5,394,703	3,957,742
Investments in subsidiaries	0	0
Property, plant and equipment, at cost:
Land	1,018,010	976,454
Buildings and improvements	4,522,855	4,003,726
Furniture and equipment	733,063	689,780
Property, plant and equipment (gross)	10,240,559	11,022,027
Less: Accumulated depreciation	(2,640,940)	(3,088,056)
Total property, plant and equipment	7,599,619	7,933,971
Total assets	12,994,322	11,891,713	11,645,760
Liabilities:
Accounts payable and accrued expenses	551,770	556,873
Notes, loans and leases payable	4,548,609	4,163,323
Financing lease liability	795,465
Operating lease liability	107,581	0
Policy benefits and losses, claims and loss expenses payable	1,015,663	1,011,183
Liabilities from investment contracts	1,753,428	1,666,742
Other policyholders' funds and liabilities	12,924	15,047
Deferred income	31,459	35,186
Deferred income taxes, net	869,671	750,970
Related party liabilities	0	0
Total liabilities	8,891,105	8,199,324
Stockholders' equity:
Additional paid-in capital	453,819	453,326
Accumulated other comprehensive loss	39,517	(66,698)
Retained earnings	4,277,034	3,976,962
Unearned employee stock ownership plan shares	0	(4,048)
Total stockholders' equity	4,103,217	3,692,389
Total liabilities and stockholders' equity	12,994,322	11,891,713
Rental Trailers and Other Rental Equipment [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross	511,872	590,039
Rental Trucks [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross	3,454,759	4,762,028
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	604,176	643,918	905,266	702,036
Reinsurance recoverables and trade receivables, net	72,170	90,832
Inventories, net	103,003	103,504
Prepaid expenses	188,780	174,100
Investments, fixed maturities and marketable equities	0	0
Investments, other	20,988	23,013
Deferred policy acquisition costs, net	0	0
Right of use Assets - Financing	67,916
Right of use Assets - Operating	1,130,473
Other assets	107,974	72,768
Related party assets	52,223	35,997
Subtotal assets	2,347,703	1,144,132
Investments in subsidiaries	657,994	534,157
Property, plant and equipment, at cost:
Land	1,018,010	976,454
Buildings and improvements	4,522,855	4,003,726
Furniture and equipment	733,063	689,780
Property, plant and equipment (gross)	10,240,559	11,022,027
Less: Accumulated depreciation	(2,640,940)	(3,088,056)
Total property, plant and equipment	7,599,619	7,933,971
Total assets	10,605,316	9,612,260
Liabilities:
Accounts payable and accrued expenses	536,387	548,099
Notes, loans and leases payable	4,548,609	4,163,323
Operating lease liability	107,581
Policy benefits and losses, claims and loss expenses payable	416,793	407,934
Liabilities from investment contracts	0	0
Other policyholders' funds and liabilities	0	0
Deferred income	31,459	35,186
Deferred income taxes, net	835,519	741,644
Related party liabilities	24,490	25,446
Total liabilities	6,500,838	5,921,632
Stockholders' equity:
Additional paid-in capital	454,029	453,536
Accumulated other comprehensive loss	40,778	(68,459)
Retained earnings	4,276,824	3,976,752
Unearned employee stock ownership plan shares	0	(4,048)
Total stockholders' equity	4,104,478	3,690,628
Total liabilities and stockholders' equity	10,605,316	9,612,260
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	511,872	590,039
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	3,454,759	4,762,028
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	5,868	5,757	5,715	6,639
Reinsurance recoverables and trade receivables, net	96,189	102,120
Inventories, net	0	0
Prepaid expenses	0	0
Investments, fixed maturities and marketable equities	290,776	279,641
Investments, other	85,760	74,679
Deferred policy acquisition costs, net	0	0
Right of use Assets - Financing	724
Right of use Assets - Operating	0
Other assets	0	2,456
Related party assets	8,007	6,639
Subtotal assets	487,324	471,292
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	487,324	471,292
Liabilities:
Accounts payable and accrued expenses	2,662	2,844
Notes, loans and leases payable	0	0
Operating lease liability	0
Policy benefits and losses, claims and loss expenses payable	223,890	229,958
Liabilities from investment contracts	0	0
Other policyholders' funds and liabilities	5,838	5,259
Deferred income	0	0
Deferred income taxes, net	10,333	6,961
Related party liabilities	5,039	3,836
Total liabilities	247,762	248,858
Stockholders' equity:
Additional paid-in capital	91,120	91,120
Accumulated other comprehensive loss	13,290	(3,721)
Retained earnings	131,851	131,734
Unearned employee stock ownership plan shares	0	0
Total stockholders' equity	239,562	222,434
Total liabilities and stockholders' equity	487,324	471,292
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	21,907	24,026	73,404	50,713
Reinsurance recoverables and trade receivables, net	31,805	31,833
Inventories, net	0	0
Prepaid expenses	0	0
Investments, fixed maturities and marketable equities	2,178,692	1,955,756
Investments, other	229,979	203,044
Deferred policy acquisition costs, net	106,354	136,276
Right of use Assets - Financing	3,181
Right of use Assets - Operating	0
Other assets	0	3,130
Related party assets	19,450	16,466
Subtotal assets	2,591,368	2,370,531
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	2,591,368	2,370,531
Liabilities:
Accounts payable and accrued expenses	12,721	5,930
Notes, loans and leases payable	0	0
Operating lease liability	0
Policy benefits and losses, claims and loss expenses payable	374,980	373,291
Liabilities from investment contracts	1,753,428	1,666,742
Other policyholders' funds and liabilities	7,086	9,788
Deferred income	0	0
Deferred income taxes, net	23,819	2,365
Related party liabilities	902	692
Total liabilities	2,172,936	2,058,808
Stockholders' equity:
Additional paid-in capital	26,271	26,271
Accumulated other comprehensive loss	85,185	(5,300)
Retained earnings	304,476	288,252
Unearned employee stock ownership plan shares	0	0
Total stockholders' equity	418,432	311,723
Total liabilities and stockholders' equity	2,591,368	2,370,531
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
Right of use Assets - Financing	0
Right of use Assets - Operating	0
Other assets	0	0
Related party assets	(31,692)	(28,213)
Subtotal assets	(31,692)	(28,213)
Investments in subsidiaries	(657,994)	(534,157)
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	(689,686)	(562,370)
Liabilities:
Accounts payable and accrued expenses	0	0
Notes, loans and leases payable	0	0
Operating lease liability	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	(30,431)	(29,974)
Total liabilities	(30,431)	(29,974)
Stockholders' equity:
Additional paid-in capital	(117,601)	(117,601)
Accumulated other comprehensive loss	(99,736)	10,782
Retained earnings	(436,117)	(419,776)
Unearned employee stock ownership plan shares	0	0
Total stockholders' equity	(659,255)	(532,396)
Total liabilities and stockholders' equity	(689,686)	(562,370)
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