Consolidating financial information by industry segment (Balance Sheets) (Details) - USD ($) $ in Thousands		Dec. 31, 2018		Mar. 31, 2018		Dec. 31, 2017		Mar. 31, 2017
ASSETS:
Cash and cash equivalents		$ 984,385		$ 759,388		$ 1,022,887		$ 697,806
Reinsurance recoverables and trade receivables, net		191,764		193,538
Inventories, net		96,187		89,877
Prepaid expenses		177,918		166,129
Investments, fixed maturities and marketable equities		2,127,342		1,919,860
Investments, other		332,532		399,064
Deferred policy acquisition costs, net		140,673		124,767
Other assets		83,839		244,782
Related party assets		38,156		33,276
Subtotal assets		4,172,796		3,930,681
Investments in subsidiaries
Property, plant and equipment, at cost:
Land		910,919		827,649
Buildings and improvements		3,762,491		3,140,713
Furniture and equipment		674,535		632,803
Property, plant and equipment, gross		10,452,724		9,537,883
Less: Accumulated depreciation		(2,979,760)		(2,721,142)
Total property, plant and equipment		7,472,964		6,816,741
Total assets		11,645,760		10,747,422		10,496,959
Liabilities:
Accounts payable and accrued expenses		517,360		511,115
Notes, loans and leases payable		3,975,764		3,513,076
Policy benefits and losses, claims and loss expenses payable		1,087,742		1,248,033
Liabilities from investment contracts		1,583,885		1,364,066
Other policyholders' funds and liabilities		12,907		10,040
Deferred income		29,226		34,276
Deferred income taxes, net		746,836		658,108
Related party liabilities
Total Liabilities		7,953,720		7,338,714
Stockholders' equity:
Additional paid-in capital		453,116		452,746
Accumulated other comprehensive loss		(74,707)		(4,623)
Retained earnings		3,985,927		3,635,561
Unearned employee stock ownership plan shares		(5,143)		(7,823)
Total stockholders' equity		3,692,040		3,408,708
Total liabilities and stockholders' equity		11,645,760		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		572,645		545,968
Rental Trucks [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross		4,532,134		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
Series A 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		905,266		702,036		978,264		671,665
Reinsurance recoverables and trade receivables, net		62,296		64,798
Inventories, net		96,187		89,877
Prepaid expenses		177,918		166,129
Investments, fixed maturities and marketable equities
Investments, other		22,988		22,992
Deferred policy acquisition costs, net
Other assets		80,205		241,493
Related party assets		41,289		40,003
Subtotal assets		1,386,149		1,327,328
Investments in subsidiaries		518,629		544,151
Property, plant and equipment, at cost:
Land		910,919		827,649
Buildings and improvements		3,762,491		3,140,713
Furniture and equipment		674,535		632,803
Property, plant and equipment, gross		10,452,724		9,537,883
Less: Accumulated depreciation		(2,979,760)		(2,721,142)
Total property, plant and equipment		7,472,964		6,816,741
Total assets		9,377,742		8,688,220
Liabilities:
Accounts payable and accrued expenses		509,060		506,158
Notes, loans and leases payable		3,975,764		3,513,076
Policy benefits and losses, claims and loss expenses payable		409,981		568,456
Liabilities from investment contracts
Other policyholders' funds and liabilities
Deferred income		29,226		34,276
Deferred income taxes, net		738,186		629,389
Related party liabilities		23,485		28,157
Total Liabilities		5,685,702		5,279,512
Stockholders' equity:
Additional paid-in capital		453,326		452,956
Accumulated other comprehensive loss		(74,707)		(4,623)
Retained earnings		3,985,717		3,635,351
Unearned employee stock ownership plan shares		(5,143)		(7,823)
Total stockholders' equity		3,692,040		3,408,708
Total liabilities and stockholders' equity		9,377,742		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		572,645		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		4,532,134		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] | Series A 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		5,715	[1]	6,639	[2]	12,296	[3]	12,725	[3]
Reinsurance recoverables and trade receivables, net		97,680	[1]	99,682	[2]
Inventories, net			[1]		[2]
Prepaid expenses			[1]		[2]
Investments, fixed maturities and marketable equities		284,871	[1]	285,846	[2]
Investments, other		69,719	[1]	65,553	[2]
Deferred policy acquisition costs, net			[1]		[2]
Other assets		532	[1]	685	[2]
Related party assets		7,359	[1]	6,959	[2]
Subtotal assets		465,876	[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		465,876	[1]	465,364	[2]
Liabilities:
Accounts payable and accrued expenses		3,745	[1]	2,582	[2]
Notes, loans and leases payable			[1]		[2]
Policy benefits and losses, claims and loss expenses payable		229,607	[1]	234,359	[2]
Liabilities from investment contracts			[1]		[2]
Other policyholders' funds and liabilities		5,050	[1]	5,377	[2]
Deferred income			[1]		[2]
Deferred income taxes, net		5,658	[1]	8,927	[2]
Related party liabilities		3,181	[1]	2,870	[2]
Total Liabilities		247,241	[1]	254,115	[2]
Stockholders' equity:
Additional paid-in capital		91,120	[1]	91,120	[2]
Accumulated other comprehensive loss		(3,026)	[1]	16,526	[2]
Retained earnings		127,240	[1]	100,302	[2]
Unearned employee stock ownership plan shares			[1]		[2]
Total stockholders' equity		218,635	[1]	211,249	[2]
Total liabilities and stockholders' equity		465,876	[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] | Series A 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		73,404	[1]	50,713	[2]	32,327	[3]	13,416	[3]
Reinsurance recoverables and trade receivables, net		31,788	[1]	29,058	[2]
Inventories, net			[1]		[2]
Prepaid expenses			[1]		[2]
Investments, fixed maturities and marketable equities		1,842,471	[1]	1,634,014	[2]
Investments, other		239,825	[1]	310,519	[2]
Deferred policy acquisition costs, net		140,673	[1]	124,767	[2]
Other assets		3,102	[1]	2,604	[2]
Related party assets		16,618	[1]	18,334	[2]
Subtotal assets		2,347,881	[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		2,347,881	[1]	2,170,009	[2]
Liabilities:
Accounts payable and accrued expenses		4,555	[1]	2,375	[2]
Notes, loans and leases payable			[1]		[2]
Policy benefits and losses, claims and loss expenses payable		448,154	[1]	445,218	[2]
Liabilities from investment contracts		1,583,885	[1]	1,364,066	[2]
Other policyholders' funds and liabilities		7,857	[1]	4,663	[2]
Deferred income			[1]		[2]
Deferred income taxes, net		2,992	[1]	19,792	[2]
Related party liabilities		444	[1]	993	[2]
Total Liabilities		2,047,887	[1]	1,837,107	[2]
Stockholders' equity:
Additional paid-in capital		26,271	[1]	26,271	[2]
Accumulated other comprehensive loss		(10,018)	[1]	35,982	[2]
Retained earnings		281,241	[1]	268,149	[2]
Unearned employee stock ownership plan shares			[1]		[2]
Total stockholders' equity		299,994	[1]	332,902	[2]
Total liabilities and stockholders' equity		2,347,881	[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] | Series A 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]	(27,110)		(32,020)
Subtotal assets		(27,110)		(32,020)
Investments in subsidiaries	[5]	(518,629)		(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		(545,739)		(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]	(27,110)		(32,020)
Total Liabilities		(27,110)		(32,020)
Stockholders' equity:
Additional paid-in capital	[5]	(117,601)		(117,601)
Accumulated other comprehensive loss	[5]	13,044		(52,508)
Retained earnings	[5]	(408,271)		(368,241)
Unearned employee stock ownership plan shares
Total stockholders' equity		(518,629)		(544,151)
Total liabilities and stockholders' equity		(545,739)		(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] | Series A 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 September 30, 2018

[2] Balances as of December 31, 2017

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

[4] Eliminate intercompany receivables and payables

[5] Eliminate investment in subsidiaries