Consolidating financial information by industry segment (Balance Sheets) (Details) - USD ($) $ in Thousands		Sep. 30, 2018		Mar. 31, 2018		Sep. 30, 2017		Mar. 31, 2017
ASSETS:
Cash and cash equivalents		$ 652,440		$ 759,388		$ 768,351		$ 697,806
Reinsurance recoverables and trade receivables, net		207,665		193,538
Inventories, net		93,071		89,877
Prepaid expenses		183,350		166,129
Investments, fixed maturities and marketable equities		2,015,402		1,919,860
Investments, other		407,774		399,064
Deferred policy acquisition costs, net		138,938		124,767
Other assets		180,475		244,782
Related party assets		30,257		33,276
Subtotal assets		3,909,372		3,930,681
Investments in subsidiaries
Property, plant and equipment, at cost:
Land		899,095		827,649
Buildings and improvements		3,581,419		3,140,713
Furniture and equipment		656,389		632,803
Property, plant and equipment (gross)		10,296,086		9,537,883
Less: Accumulated depreciation		(2,881,613)		(2,721,142)
Total property, plant and equipment		7,414,473		6,816,741
Total assets		11,323,845		10,747,422		10,082,890
Liabilities:
Accounts payable and accrued expenses		535,185		511,115
Notes, loans and leases payable		3,686,572		3,513,076
Policy benefits and losses, claims and loss expenses payable		1,198,053		1,248,033
Liabilities from investment contracts		1,490,172		1,364,066
Other policyholders' funds and liabilities		13,342		10,040
Deferred income		34,136		34,276
Deferred income taxes, net		733,218		658,108
Related party liabilities
Total liabilities		7,690,678		7,338,714
Stockholders' equity:
Additional paid-in capital		453,006		452,746
Accumulated other comprehensive loss		(62,238)		(4,623)
Retained earnings		3,917,087		3,635,561
Unearned employee stock ownership plan shares		(7,535)		(7,823)
Total stockholders' equity		3,633,167		3,408,708
Total liabilities and stockholders' equity		11,323,845		10,747,422
Rental Trailers and Other Rental Equipment [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross		562,167		545,968
Rental Trucks [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross		4,597,016		4,390,750
Series A Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued
Series B Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued
Serial Common Stock [Member]
Stockholders' equity:
Common stock, value, issued
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		618,163		702,036		745,923		671,665
Reinsurance recoverables and trade receivables, net		78,630		64,798
Inventories, net		93,071		89,877
Prepaid expenses		183,350		166,129
Investments, fixed maturities and marketable equities
Investments, other		22,749		22,992
Deferred policy acquisition costs, net
Other assets		176,628		241,493
Related party assets		32,155		40,003
Subtotal assets		1,204,746		1,327,328
Investments in subsidiaries		512,244		544,151
Property, plant and equipment, at cost:
Land		899,095		827,649
Buildings and improvements		3,581,419		3,140,713
Furniture and equipment		656,389		632,803
Property, plant and equipment (gross)		10,296,086		9,537,883
Less: Accumulated depreciation		(2,881,613)		(2,721,142)
Total property, plant and equipment		7,414,473		6,816,741
Total assets		9,131,463		8,688,220
Liabilities:
Accounts payable and accrued expenses		520,882		506,158
Notes, loans and leases payable		3,686,572		3,513,076
Policy benefits and losses, claims and loss expenses payable		513,370		568,456
Liabilities from investment contracts
Other policyholders' funds and liabilities
Deferred income		34,136		34,276
Deferred income taxes, net		718,757		629,389
Related party liabilities		24,579		28,157
Total liabilities		5,498,296		5,279,512
Stockholders' equity:
Additional paid-in capital		453,216		452,956
Accumulated other comprehensive loss		(62,238)		(4,623)
Retained earnings		3,916,877		3,635,351
Unearned employee stock ownership plan shares		(7,535)		(7,823)
Total stockholders' equity		3,633,167		3,408,708
Total liabilities and stockholders' equity		9,131,463		8,688,220
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		562,167		545,968
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		4,597,016		4,390,750
Operating Segments [Member] | Moving and Storage Consolidations [Member] | Series A Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued
Operating Segments [Member] | Moving and Storage Consolidations [Member] | Series B Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued
Operating Segments [Member] | Moving and Storage Consolidations [Member] | Serial Common Stock [Member]
Stockholders' equity:
Common stock, value, issued
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		7,247	[1]	6,639	[2]	10,791	[3]	12,725	[3]
Reinsurance recoverables and trade receivables, net		98,531	[1]	99,682	[2]
Inventories, net			[1]		[2]
Prepaid expenses			[1]		[2]
Investments, fixed maturities and marketable equities		282,811	[1]	285,846	[2]
Investments, other		67,253	[1]	65,553	[2]
Deferred policy acquisition costs, net			[1]		[2]
Other assets		762	[1]	685	[2]
Related party assets		8,544	[1]	6,959	[2]
Subtotal assets		465,148	[1]	465,364	[2]
Investments in subsidiaries			[1]		[2]
Property, plant and equipment, at cost:
Land			[1]		[2]
Buildings and improvements			[1]		[2]
Furniture and equipment			[1]		[2]
Property, plant and equipment (gross)			[1]		[2]
Less: Accumulated depreciation			[1]		[2]
Total property, plant and equipment			[1]		[2]
Total assets		465,148	[1]	465,364	[2]
Liabilities:
Accounts payable and accrued expenses		3,688	[1]	2,582	[2]
Notes, loans and leases payable			[1]		[2]
Policy benefits and losses, claims and loss expenses payable		235,524	[1]	234,359	[2]
Liabilities from investment contracts			[1]		[2]
Other policyholders' funds and liabilities		5,391	[1]	5,377	[2]
Deferred income			[1]		[2]
Deferred income taxes, net		6,648	[1]	8,927	[2]
Related party liabilities		2,388	[1]	2,870	[2]
Total liabilities		253,639	[1]	254,115	[2]
Stockholders' equity:
Additional paid-in capital		91,120	[1]	91,120	[2]
Accumulated other comprehensive loss		(1,337)	[1]	16,526	[2]
Retained earnings		118,425	[1]	100,302	[2]
Unearned employee stock ownership plan shares			[1]		[2]
Total stockholders' equity		211,509	[1]	211,249	[2]
Total liabilities and stockholders' equity		465,148	[1]	465,364	[2]
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			[1]		[2]
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			[1]		[2]
Operating Segments [Member] | Property and Casualty Insurance [Member] | Series A Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued			[1]		[2]
Operating Segments [Member] | Property and Casualty Insurance [Member] | Series B Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued			[1]		[2]
Operating Segments [Member] | Property and Casualty Insurance [Member] | Serial Common Stock [Member]
Stockholders' equity:
Common stock, value, issued			[1]		[2]
Operating Segments [Member] | Property and Casualty Insurance [Member] | Amerco Common Stock [Member]
Stockholders' equity:
Common stock, value, issued		3,301	[1]	3,301	[2]
Operating Segments [Member] | Property and Casualty Insurance [Member] | Common Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value			[1]		[2]
Operating Segments [Member] | Property and Casualty Insurance [Member] | Preferred Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value			[1]		[2]
Operating Segments [Member] | Life Insurance [Member]
ASSETS:
Cash and cash equivalents		27,030	[1]	50,713	[2]	11,637	[3]	13,416	[3]
Reinsurance recoverables and trade receivables, net		30,504	[1]	29,058	[2]
Inventories, net			[1]		[2]
Prepaid expenses			[1]		[2]
Investments, fixed maturities and marketable equities		1,732,591	[1]	1,634,014	[2]
Investments, other		317,772	[1]	310,519	[2]
Deferred policy acquisition costs, net		138,938	[1]	124,767	[2]
Other assets		3,085	[1]	2,604	[2]
Related party assets		16,902	[1]	18,334	[2]
Subtotal assets		2,266,822	[1]	2,170,009	[2]
Investments in subsidiaries			[1]		[2]
Property, plant and equipment, at cost:
Land			[1]		[2]
Buildings and improvements			[1]		[2]
Furniture and equipment			[1]		[2]
Property, plant and equipment (gross)			[1]		[2]
Less: Accumulated depreciation			[1]		[2]
Total property, plant and equipment			[1]		[2]
Total assets		2,266,822	[1]	2,170,009	[2]
Liabilities:
Accounts payable and accrued expenses		10,615	[1]	2,375	[2]
Notes, loans and leases payable			[1]		[2]
Policy benefits and losses, claims and loss expenses payable		449,159	[1]	445,218	[2]
Liabilities from investment contracts		1,490,172	[1]	1,364,066	[2]
Other policyholders' funds and liabilities		7,951	[1]	4,663	[2]
Deferred income			[1]		[2]
Deferred income taxes, net		7,813	[1]	19,792	[2]
Related party liabilities		377	[1]	993	[2]
Total liabilities		1,966,087	[1]	1,837,107	[2]
Stockholders' equity:
Additional paid-in capital		26,271	[1]	26,271	[2]
Accumulated other comprehensive loss		(3,428)	[1]	35,982	[2]
Retained earnings		275,392	[1]	268,149	[2]
Unearned employee stock ownership plan shares			[1]		[2]
Total stockholders' equity		300,735	[1]	332,902	[2]
Total liabilities and stockholders' equity		2,266,822	[1]	2,170,009	[2]
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			[1]		[2]
Operating Segments [Member] | Life Insurance [Member] | Rental Trucks [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross			[1]		[2]
Operating Segments [Member] | Life Insurance [Member] | Series A Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued			[1]		[2]
Operating Segments [Member] | Life Insurance [Member] | Series B Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued			[1]		[2]
Operating Segments [Member] | Life Insurance [Member] | Serial Common Stock [Member]
Stockholders' equity:
Common stock, value, issued			[1]		[2]
Operating Segments [Member] | Life Insurance [Member] | Amerco Common Stock [Member]
Stockholders' equity:
Common stock, value, issued		2,500	[1]	2,500	[2]
Operating Segments [Member] | Life Insurance [Member] | Common Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value			[1]		[2]
Operating Segments [Member] | Life Insurance [Member] | Preferred Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value			[1]		[2]
Consolidation, Eliminations [Member]
ASSETS:
Cash and cash equivalents
Reinsurance recoverables and trade receivables, net
Inventories, net
Prepaid expenses
Investments, fixed maturities and marketable equities
Investments, other
Deferred policy acquisition costs, net
Other assets
Related party assets	[4]	(27,344)		(32,020)
Subtotal assets		(27,344)		(32,020)
Investments in subsidiaries	[5]	(512,244)		(544,151)
Property, plant and equipment, at cost:
Land
Buildings and improvements
Furniture and equipment
Property, plant and equipment (gross)
Less: Accumulated depreciation
Total property, plant and equipment
Total assets		(539,588)		(576,171)
Liabilities:
Accounts payable and accrued expenses
Notes, loans and leases payable
Policy benefits and losses, claims and loss expenses payable
Liabilities from investment contracts
Other policyholders' funds and liabilities
Deferred income
Deferred income taxes, net
Related party liabilities	[4]	(27,344)		(32,020)
Total liabilities		(27,344)		(32,020)
Stockholders' equity:
Additional paid-in capital	[5]	(117,601)		(117,601)
Accumulated other comprehensive loss	[5]	4,765		(52,508)
Retained earnings	[5]	(393,607)		(368,241)
Unearned employee stock ownership plan shares
Total stockholders' equity		(512,244)		(544,151)
Total liabilities and stockholders' equity		(539,588)		(576,171)
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
Consolidation, Eliminations [Member] | Rental Trucks [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross
Consolidation, Eliminations [Member] | Series A Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued
Consolidation, Eliminations [Member] | Series B Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued
Consolidation, Eliminations [Member] | Serial Common Stock [Member]
Stockholders' equity:
Common stock, value, issued
Consolidation, Eliminations [Member] | Amerco Common Stock [Member]
Stockholders' equity:
Common stock, value, issued	[5]	(5,801)		(5,801)
Consolidation, Eliminations [Member] | Common Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value
Consolidation, Eliminations [Member] | Preferred Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value

[1] Balances as of June 30, 2018

[2] Balances as of December 31, 2017

[3] Balance for the period ended June 30, 2017

[4] Eliminate intercompany receivables and payables

[5] Eliminate investment in subsidiaries