Consolidating financial information by industry segment (Balance Sheets) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Mar. 31, 2017
ASSETS:
Cash and cash equivalents	$ 494,352		$ 673,701		$ 759,388	$ 697,806
Reinsurance recoverables and trade receivables, net	186,672		224,785
Inventories, net	101,083		103,504
Prepaid expenses	562,904		174,100
Investments, fixed maturities and marketable equities	2,492,738		2,235,397
Investments, other	360,373		300,736
Deferred policy acquisition costs, net	103,118		136,276
Other assets	71,956		78,354
Right of use assets, financing, net	1,080,353		0
Right of use assets, operating	106,631		0
Related party assets	34,784		30,889
Subtotal assets	5,594,964		3,957,742
Investments in subsidiaries	0		0
Property, plant and equipment, at cost:
Land	1,032,945		976,454
Buildings and improvements	4,663,461		4,003,726
Furniture and equipment	752,363		689,780
Property, plant and equipment (gross)	10,556,222		11,022,027
Less: Accumulated depreciation	(2,713,162)		(3,088,056)
Total property, plant and equipment	7,843,060		7,933,971
Total assets	13,438,024		11,891,713		10,747,422
Liabilities:
Accounts payable and accrued expenses	554,353		556,873
Notes, loans and leases payable	4,621,291		4,163,323
Operating lease liabilities	106,443		0
Policy benefits and losses, claims and loss expenses payable	997,647		1,011,183
Liabilities from investment contracts	1,802,217		1,666,742
Other policyholders' funds and liabilities	10,190		15,047
Deferred income	31,620		35,186
Deferred income taxes	1,093,543		750,970
Related party liabilities	0		0
Total liabilities	9,217,304		8,199,324
Stockholders' equity:
Additional paid-in capital	453,819		453,326
Accumulated other comprehensive loss	34,652		(66,698)
Retained earnings	4,399,402		3,976,962
Unearned employee stock ownership plan shares	0		(4,048)
Total stockholders' equity	4,220,720		3,692,389
Total liabilities and stockholders' equity	13,438,024		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	511,520		590,039
Rental Trucks [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross	3,595,933		4,762,028
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	459,078		643,918		702,036	671,665
Reinsurance recoverables and trade receivables, net	60,073		90,832
Inventories, net	101,083		103,504
Prepaid expenses	562,904		174,100
Investments, fixed maturities and marketable equities	0		0
Investments, other	20,988		23,013
Deferred policy acquisition costs, net	0		0
Other assets	69,128		72,768
Right of use assets, financing, net	1,080,353
Right of use assets, operating	106,631
Related party assets	41,027		35,997
Subtotal assets	2,501,265		1,144,132
Investments in subsidiaries	668,498		534,157
Property, plant and equipment, at cost:
Land	1,032,945		976,454
Buildings and improvements	4,663,461		4,003,726
Furniture and equipment	752,363		689,780
Property, plant and equipment (gross)	10,556,222		11,022,027
Less: Accumulated depreciation	(2,713,162)		(3,088,056)
Total property, plant and equipment	7,843,060		7,933,971
Total assets	11,012,823		9,612,260
Liabilities:
Accounts payable and accrued expenses	545,685		548,099
Notes, loans and leases payable	4,609,844		4,163,323
Operating lease liabilities	106,443
Policy benefits and losses, claims and loss expenses payable	410,107		407,934
Liabilities from investment contracts	0		0
Other policyholders' funds and liabilities	0		0
Deferred income	31,620		35,186
Deferred income taxes	1,063,681		741,644
Related party liabilities	24,275		25,446
Total liabilities	6,791,655		5,921,632
Stockholders' equity:
Additional paid-in capital	454,029		453,536
Accumulated other comprehensive loss	35,100		(68,459)
Retained earnings	4,399,192		3,976,752
Unearned employee stock ownership plan shares	0		(4,048)
Total stockholders' equity	4,221,168		3,690,628
Total liabilities and stockholders' equity	11,012,823		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	511,520		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	3,595,933		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] | 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	4,794		5,757		6,639	12,725
Reinsurance recoverables and trade receivables, net	93,995		102,120
Inventories, net	0		0
Prepaid expenses	0		0
Investments, fixed maturities and marketable equities	288,998		279,641
Investments, other	90,145		74,679
Deferred policy acquisition costs, net	0		0
Other assets	680		2,456
Right of use assets, financing, net	0
Right of use assets, operating	0
Related party assets	7,137		6,639
Subtotal assets	485,749		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	485,749		471,292
Liabilities:
Accounts payable and accrued expenses	5,530		2,844
Notes, loans and leases payable	0		0
Operating lease liabilities	0
Policy benefits and losses, claims and loss expenses payable	210,341	$ 210,341	229,958	$ 229,958
Liabilities from investment contracts	0		0
Other policyholders' funds and liabilities	5,751		5,259
Deferred income	0		0
Deferred income taxes	8,447		6,961
Related party liabilities	4,616		3,836
Total liabilities	234,685		248,858
Stockholders' equity:
Additional paid-in capital	91,120		91,120
Accumulated other comprehensive loss	12,581		(3,721)
Retained earnings	144,062		131,734
Unearned employee stock ownership plan shares	0		0
Total stockholders' equity	251,064		222,434
Total liabilities and stockholders' equity	485,749		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] | 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	30,480		24,026		50,713	13,416
Reinsurance recoverables and trade receivables, net	32,604		31,833
Inventories, net	0		0
Prepaid expenses	0		0
Investments, fixed maturities and marketable equities	2,203,740		1,955,756
Investments, other	249,240		203,044
Deferred policy acquisition costs, net	103,118		136,276
Other assets	2,148		3,130
Right of use assets, financing, net	0
Right of use assets, operating	0
Related party assets	18,629		16,466
Subtotal assets	2,639,959		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	2,639,959		2,370,531
Liabilities:
Accounts payable and accrued expenses	3,138		5,930
Notes, loans and leases payable	11,447		0
Operating lease liabilities	0
Policy benefits and losses, claims and loss expenses payable	377,199		373,291
Liabilities from investment contracts	1,802,217		1,666,742
Other policyholders' funds and liabilities	4,439		9,788
Deferred income	0		0
Deferred income taxes	21,415		2,365
Related party liabilities	2,670		692
Total liabilities	2,222,525		2,058,808
Stockholders' equity:
Additional paid-in capital	26,271		26,271
Accumulated other comprehensive loss	78,550		(5,300)
Retained earnings	310,113		288,252
Unearned employee stock ownership plan shares	0		0
Total stockholders' equity	417,434		311,723
Total liabilities and stockholders' equity	2,639,959		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] | 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
Right of use assets, operating	0
Related party assets	(32,009)		(28,213)
Subtotal assets	(32,009)		(28,213)
Investments in subsidiaries	(668,498)		(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	(700,507)		(562,370)
Liabilities:
Accounts payable and accrued expenses	0		0
Notes, loans and leases payable	0		0
Operating lease liabilities	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	(31,561)		(29,974)
Total liabilities	(31,561)		(29,974)
Stockholders' equity:
Additional paid-in capital	(117,601)		(117,601)
Accumulated other comprehensive loss	(91,579)		10,782
Retained earnings	(453,965)		(419,776)
Unearned employee stock ownership plan shares	0		0
Total stockholders' equity	(668,946)		(532,396)
Total liabilities and stockholders' equity	(700,507)		(562,370)
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