Consolidating financial information by industry segment (Balance Sheets) (Details) - USD ($) $ in Thousands		Mar. 31, 2019		Dec. 31, 2018	Mar. 31, 2018		Dec. 31, 2017	Mar. 31, 2017		Mar. 31, 2016
ASSETS:
Cash and cash equivalents		$ 673,701			$ 759,388			$ 697,806		$ 600,646
Reinsurance recoverables and trade receivables, net		224,785			193,538
Inventories, net		103,504			89,877
Prepaid expenses		174,100			166,129
Investments, fixed maturities and marketable equities		2,235,397			1,919,860
Investments, other		300,736			399,064
Deferred policy acquisition costs, net		136,276			124,767
Other assets		78,354			244,782
Related party assets		30,889			33,276
Subtotal assets		3,957,742			3,930,681
Investments in subsidiaries
Property, plant and equipment, at cost:
Land		976,454			827,649
Buildings and improvements		4,003,726			3,140,713
Furniture and equipment		689,780			632,803
Property, plant and equipment (gross)		11,022,027			9,537,883
Less: Accumulated depreciation		(3,088,056)			(2,721,142)
Total property, plant and equipment		7,933,971			6,816,741
Total assets		11,891,713			10,747,422			9,405,840
Liabilities:
Accounts payable and accrued expenses		556,873			511,115
Notes, loans and leases payable		4,163,323			3,513,076
Policy benefits and losses, claims and loss expenses payable		1,011,183			1,248,033
Liabilities from investment contracts		1,666,742			1,364,066
Other policyholders' funds and liabilities		15,047			10,040
Deferred income		35,186			34,276
Deferred income taxes		750,970			658,108
Related party liabilities
Total liabilities		8,199,324			7,338,714
Stockholders' equity:
Additional paid-in capital		453,326			452,746
Accumulated other comprehensive loss		(66,698)			(4,623)
Retained earnings		3,976,962			3,635,561
Unearned employee stock ownership plan shares		(4,048)			(7,823)
Total stockholders' equity		3,692,389			3,408,708
Total liabilities and stockholders' equity		11,891,713			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		590,039			545,968
Rental Trucks [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross		4,762,028			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		(525,663)			(525,653)
Preferred Stock in Treasury [Member]
Stockholders' equity:
Treasury stock, value		(151,997)			(151,997)
Property and Casualty Insurance [Member]
Liabilities:
Policy benefits and losses, claims and loss expenses payable				$ 229,958			$ 234,359
Operating Segments [Member] | Moving and Storage Consolidations [Member]
ASSETS:
Cash and cash equivalents		643,918			702,036			671,665		585,666
Reinsurance recoverables and trade receivables, net		90,832			64,798
Inventories, net		103,504			89,877
Prepaid expenses		174,100			166,129
Investments, fixed maturities and marketable equities
Investments, other		23,013			22,992
Deferred policy acquisition costs, net
Other assets		72,768			241,493
Related party assets		35,997			40,003
Subtotal assets		1,144,132			1,327,328
Investments in subsidiaries		534,157			544,151
Property, plant and equipment, at cost:
Land		976,454			827,649
Buildings and improvements		4,003,726			3,140,713
Furniture and equipment		689,780			632,803
Property, plant and equipment (gross)		11,022,027			9,537,883
Less: Accumulated depreciation		(3,088,056)			(2,721,142)
Total property, plant and equipment		7,933,971			6,816,741
Total assets		9,612,260			8,688,220
Liabilities:
Accounts payable and accrued expenses		548,099			506,158
Notes, loans and leases payable		4,163,323			3,513,076
Policy benefits and losses, claims and loss expenses payable		407,934			568,456
Liabilities from investment contracts
Other policyholders' funds and liabilities
Deferred income		35,186			34,276
Deferred income taxes		741,644			629,389
Related party liabilities		25,446			28,157
Total liabilities		5,921,632			5,279,512
Stockholders' equity:
Additional paid-in capital		453,536			452,956
Accumulated other comprehensive loss		(68,459)			(4,623)
Retained earnings		3,976,752			3,635,351
Unearned employee stock ownership plan shares		(4,048)			(7,823)
Total stockholders' equity		3,690,628			3,408,708
Total liabilities and stockholders' equity		9,612,260			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		590,039			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		4,762,028			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		5,757	[1]		6,639	[2]		12,725	[3]	14,049	[3]
Reinsurance recoverables and trade receivables, net		102,120	[1]		99,682	[2]
Inventories, net			[1]			[2]
Prepaid expenses			[1]			[2]
Investments, fixed maturities and marketable equities		279,641	[1]		285,846	[2]
Investments, other		74,679	[1]		65,553	[2]
Deferred policy acquisition costs, net			[1]			[2]
Other assets		2,456	[1]		685	[2]
Related party assets		6,639	[1]		6,959	[2]
Subtotal assets		471,292	[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		471,292	[1]		465,364	[2]
Liabilities:
Accounts payable and accrued expenses		2,844	[1]		2,582	[2]
Notes, loans and leases payable			[1]			[2]
Policy benefits and losses, claims and loss expenses payable		229,958	[1]		234,359	[2]
Liabilities from investment contracts			[1]			[2]
Other policyholders' funds and liabilities		5,259	[1]		5,377	[2]
Deferred income			[1]			[2]
Deferred income taxes		6,961	[1]		8,927	[2]
Related party liabilities		3,836	[1]		2,870	[2]
Total liabilities		248,858	[1]		254,115	[2]
Stockholders' equity:
Additional paid-in capital		91,120	[1]		91,120	[2]
Accumulated other comprehensive loss		(3,721)	[1]		16,526	[2]
Retained earnings		131,734	[1]		100,302	[2]
Unearned employee stock ownership plan shares			[1]			[2]
Total stockholders' equity		222,434	[1]		211,249	[2]
Total liabilities and stockholders' equity		471,292	[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		24,026	[1]		50,713	[2]		$ 13,416	[3]	$ 931	[3]
Reinsurance recoverables and trade receivables, net		31,833	[1]		29,058	[2]
Inventories, net			[1]			[2]
Prepaid expenses			[1]			[2]
Investments, fixed maturities and marketable equities		1,955,756	[1]		1,634,014	[2]
Investments, other		203,044	[1]		310,519	[2]
Deferred policy acquisition costs, net		136,276	[1]		124,767	[2]
Other assets		3,130	[1]		2,604	[2]
Related party assets		16,466	[1]		18,334	[2]
Subtotal assets		2,370,531	[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		2,370,531	[1]		2,170,009	[2]
Liabilities:
Accounts payable and accrued expenses		5,930	[1]		2,375	[2]
Notes, loans and leases payable			[1]			[2]
Policy benefits and losses, claims and loss expenses payable		373,291	[1]		445,218	[2]
Liabilities from investment contracts		1,666,742	[1]		1,364,066	[2]
Other policyholders' funds and liabilities		9,788	[1]		4,663	[2]
Deferred income			[1]			[2]
Deferred income taxes		2,365	[1]		19,792	[2]
Related party liabilities		692	[1]		993	[2]
Total liabilities		2,058,808	[1]		1,837,107	[2]
Stockholders' equity:
Additional paid-in capital		26,271	[1]		26,271	[2]
Accumulated other comprehensive loss		(5,300)	[1]		35,982	[2]
Retained earnings		288,252	[1]		268,149	[2]
Unearned employee stock ownership plan shares			[1]			[2]
Total stockholders' equity		311,723	[1]		332,902	[2]
Total liabilities and stockholders' equity		2,370,531	[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]
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]	(28,213)			(32,020)
Subtotal assets		(28,213)			(32,020)
Investments in subsidiaries	[5]	(534,157)			(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		(562,370)			(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
Related party liabilities	[4]	(29,974)			(32,020)
Total liabilities		(29,974)			(32,020)
Stockholders' equity:
Additional paid-in capital	[5]	(117,601)			(117,601)
Accumulated other comprehensive loss	[5]	10,782			(52,508)
Retained earnings	[5]	(419,776)			(368,241)
Unearned employee stock ownership plan shares
Total stockholders' equity		(532,396)			(544,151)
Total liabilities and stockholders' equity		(562,370)			(576,171)
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, 2018

[2] Balances as of December 31, 2016

[3] Balance for the period ended December 31, 2016

[4] Eliminate intercompany receivables and payables

[5] Eliminate investment in subsidiaries