Consolidating financial information by industry segment (Balance Sheets) (Details) - USD ($) $ in Thousands		Mar. 31, 2018		Dec. 31, 2017	Mar. 31, 2017		Dec. 31, 2016	Mar. 31, 2016		Mar. 31, 2015
ASSETS:
Cash and cash equivalents		$ 759,388			$ 697,806			$ 600,646		$ 441,850
Reinsurance recoverables and trade receivables, net		193,538			178,081
Inventories, net		89,877			82,439
Prepaid expenses		165,692			124,728
Investments, fixed maturities and marketable equities		1,919,860			1,663,768
Investments, other		399,064			367,830
Deferred policy acquisition costs, net		124,767			130,213
Other assets		244,782			97,525
Related party assets		33,276			86,168
Subtotal assets		3,930,244			3,428,558
Investments in subsidiaries
Property, plant and equipment, at cost:
Land		827,649			648,757
Buildings and improvements		3,140,713			2,618,265
Furniture and equipment		632,803			510,415
Property, plant and equipment (gross)		9,537,883			8,361,315
Less: Accumulated depreciation		(2,721,142)			(2,384,033)
Total property, plant and equipment		6,816,741			5,977,282
Total assets		10,746,985			9,405,840			8,109,288
Liabilities:
Accounts payable and accrued expenses		510,678			450,541
Notes, loans and leases payable		3,513,076			3,262,880
Policy benefits and losses, claims and loss expenses payable		1,248,033			1,086,322
Liabilities from investment contracts		1,364,066			1,112,498
Other policyholders' funds and liabilities		10,040			10,150
Deferred income		34,276			28,696
Deferred income taxes		658,108			835,009
Related party liabilities
Total liabilities		7,338,277			6,786,096
Stockholders' equity:
Additional paid-in capital		452,746			452,172
Accumulated other comprehensive loss		(4,623)			(51,236)
Retained earnings		3,635,561			2,892,893
Unearned employee stock ownership plan shares		(7,823)			(6,932)
Total stockholders' equity		3,408,708			2,619,744
Total liabilities and stockholders' equity		10,746,985			9,405,840
Rental Trailers and Other Rental Equipment [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross		545,968			492,280
Rental Trucks [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross		4,390,750			4,091,598
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		(525,663)			(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		702,036			671,665			585,666		431,873
Reinsurance recoverables and trade receivables, net		64,798			41,234
Inventories, net		89,877			82,439
Prepaid expenses		165,692			124,728
Investments, fixed maturities and marketable equities
Investments, other		22,992			35,342
Deferred policy acquisition costs, net
Other assets		241,493			93,197
Related party assets		40,003			88,829
Subtotal assets		1,326,891			1,137,434
Investments in subsidiaries		544,151			477,058
Property, plant and equipment, at cost:
Land		827,649			648,757
Buildings and improvements		3,140,713			2,618,265
Furniture and equipment		632,803			510,415
Property, plant and equipment (gross)		9,537,883			8,361,315
Less: Accumulated depreciation		(2,721,142)			(2,384,033)
Total property, plant and equipment		6,816,741			5,977,282
Total assets		8,687,783			7,591,774
Liabilities:
Accounts payable and accrued expenses		505,721			441,667
Notes, loans and leases payable		3,513,076			3,262,880
Policy benefits and losses, claims and loss expenses payable		568,456			399,181
Liabilities from investment contracts
Other policyholders' funds and liabilities
Deferred income		34,276			28,696
Deferred income taxes		629,389			809,566
Related party liabilities		28,157			30,040
Total liabilities		5,279,075			4,972,030
Stockholders' equity:
Additional paid-in capital		452,956			452,382
Accumulated other comprehensive loss		(4,623)			(51,236)
Retained earnings		3,635,351			2,892,683
Unearned employee stock ownership plan shares		(7,823)			(6,932)
Total stockholders' equity		3,408,708			2,619,744
Total liabilities and stockholders' equity		8,687,783			7,591,774
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		545,968			492,280
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		4,390,750			4,091,598
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		6,639	[1]		12,725	[2]		14,049	[3]	8,495	[3]
Reinsurance recoverables and trade receivables, net		99,682	[1]		107,757	[2]
Inventories, net			[1]			[2]
Prepaid expenses			[1]			[2]
Investments, fixed maturities and marketable equities		285,846	[1]		248,816	[2]
Investments, other		65,553	[1]		63,086	[2]
Deferred policy acquisition costs, net			[1]			[2]
Other assets		685	[1]		1,922	[2]
Related party assets		6,959	[1]		11,496	[2]
Subtotal assets		465,364	[1]		445,802	[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		465,364	[1]		445,802	[2]
Liabilities:
Accounts payable and accrued expenses		2,582	[1]		1,926	[2]
Notes, loans and leases payable			[1]			[2]
Policy benefits and losses, claims and loss expenses payable		234,359	[1]	$ 234,359	244,980	[2]	$ 244,980
Liabilities from investment contracts			[1]			[2]
Other policyholders' funds and liabilities		5,377	[1]		4,184	[2]
Deferred income			[1]			[2]
Deferred income taxes		8,927	[1]		11,243	[2]
Related party liabilities		2,870	[1]		2,539	[2]
Total liabilities		254,115	[1]		264,872	[2]
Stockholders' equity:
Additional paid-in capital		91,120	[1]		91,120	[2]
Accumulated other comprehensive loss		16,526	[1]		6,166	[2]
Retained earnings		100,302	[1]		80,343	[2]
Unearned employee stock ownership plan shares			[1]			[2]
Total stockholders' equity		211,249	[1]		180,930	[2]
Total liabilities and stockholders' equity		465,364	[1]		445,802	[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		50,713	[1]		13,416	[2]		$ 931	[3]	$ 1,482	[3]
Reinsurance recoverables and trade receivables, net		29,058	[1]		29,090	[2]
Inventories, net			[1]			[2]
Prepaid expenses			[1]			[2]
Investments, fixed maturities and marketable equities		1,634,014	[1]		1,414,952	[2]
Investments, other		310,519	[1]		269,402	[2]
Deferred policy acquisition costs, net		124,767	[1]		130,213	[2]
Other assets		2,604	[1]		2,406	[2]
Related party assets		18,334	[1]		18,465	[2]
Subtotal assets		2,170,009	[1]		1,877,944	[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		2,170,009	[1]		1,877,944	[2]
Liabilities:
Accounts payable and accrued expenses		2,375	[1]		6,948	[2]
Notes, loans and leases payable			[1]			[2]
Policy benefits and losses, claims and loss expenses payable		445,218	[1]		442,161	[2]
Liabilities from investment contracts		1,364,066	[1]		1,112,498	[2]
Other policyholders' funds and liabilities		4,663	[1]		5,966	[2]
Deferred income			[1]			[2]
Deferred income taxes		19,792	[1]		14,200	[2]
Related party liabilities		993	[1]		43	[2]
Total liabilities		1,837,107	[1]		1,581,816	[2]
Stockholders' equity:
Additional paid-in capital		26,271	[1]		26,271	[2]
Accumulated other comprehensive loss		35,982	[1]		16,933	[2]
Retained earnings		268,149	[1]		250,424	[2]
Unearned employee stock ownership plan shares			[1]			[2]
Total stockholders' equity		332,902	[1]		296,128	[2]
Total liabilities and stockholders' equity		2,170,009	[1]		1,877,944	[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]
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]	(32,020)			(32,622)
Subtotal assets		(32,020)			(32,622)
Investments in subsidiaries	[5]	(544,151)			(477,058)
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		(576,171)			(509,680)
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
Related party liabilities	[4]	(32,020)			(32,622)
Total liabilities		(32,020)			(32,622)
Stockholders' equity:
Additional paid-in capital	[5]	(117,601)			(117,601)
Accumulated other comprehensive loss	[5]	(52,508)			(23,099)
Retained earnings	[5]	(368,241)			(330,557)
Unearned employee stock ownership plan shares
Total stockholders' equity		(544,151)			(477,058)
Total liabilities and stockholders' equity		(576,171)			(509,680)
Eliminations [Member] | Rental Trailers and Other Rental Equipment [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross
Eliminations [Member] | Rental Trucks [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross
Eliminations [Member] | Series A Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued
Eliminations [Member] | Series B Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value, issued
Eliminations [Member] | Series A Common Stock [Member]
Stockholders' equity:
Common stock, value, issued
Eliminations [Member] | Amerco Common Stock [Member]
Stockholders' equity:
Common stock, value, issued	[5]	(5,801)			(5,801)
Eliminations [Member] | Common Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value
Eliminations [Member] | Preferred Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value

[1] Balances as of December 31, 2016

[2] Balances as of December 31, 2015

[3] Balance for the period ended December 31, 2014

[4] Eliminate intercompany receivables and payables

[5] Eliminate investment in subsidiaries