Consolidating financial information by industry segment (Balance Sheets) (Details) - USD ($) $ in Thousands		Dec. 31, 2017		Mar. 31, 2017		Dec. 31, 2016		Mar. 31, 2016
ASSETS:
Cash and cash equivalents		$ 1,022,887		$ 697,806		$ 984,185		$ 600,646
Reinsurance recoverables and trade receivables, net		205,873		178,081
Inventories, net		95,672		82,439
Prepaid expenses		147,375		124,728
Investments, fixed maturities and marketable equities		1,868,314		1,663,768
Investments, other		407,822		367,830
Deferred policy acquisition costs, net		124,820		130,213
Other assets		93,608		97,525
Related party assets		42,909		86,168
Subtotal assets		4,009,280		3,428,558
Investments in subsidiaries
Property, plant and equipment, at cost:
Land		738,525		648,757
Buildings and improvements		3,000,208		2,618,265
Furniture and equipment		600,216		510,415
Property, plant and equipment, gross		9,120,588		8,361,315
Less: Accumulated depreciation		(2,632,909)		(2,384,033)
Total property, plant and equipment		6,487,679		5,977,282
Total assets		10,496,959		9,405,840		9,287,317
Liabilities:
Accounts payable and accrued expenses		469,955		450,541
Notes, loans and leases payable		3,493,908		3,262,880
Policy benefits and losses, claims and loss expenses payable		1,090,206		1,086,322
Liabilities from investment contracts		1,319,945		1,112,498
Other policyholders' funds and liabilities		10,739		10,150
Deferred income		27,491		28,696
Deferred income taxes, net		671,548		835,009
Related party liabilities
Total Liabilities		7,083,792		6,786,096
Stockholders' equity:
Additional paid-in capital		452,619		452,172
Accumulated other comprehensive loss		(9,911)		(51,236)
Retained earnings		3,643,253		2,892,893
Unearned employee stock ownership plan shares		(5,641)		(6,932)
Total stockholders' equity		3,413,167		2,619,744
Total liabilities and stockholders' equity		10,496,959		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		538,334		492,280
Rental Trucks [Member]
Property, plant and equipment, at cost:
Property subject to or available for operating lease, gross		4,243,305		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		(525,653)		(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		978,264		671,665		971,002		585,666
Reinsurance recoverables and trade receivables, net		74,264		41,234
Inventories, net		95,672		82,439
Prepaid expenses		147,375		124,728
Investments, fixed maturities and marketable equities
Investments, other		23,144		35,342
Deferred policy acquisition costs, net
Other assets		90,442		93,197
Related party assets		45,111		88,829
Subtotal assets		1,454,272		1,137,434
Investments in subsidiaries		522,874		477,058
Property, plant and equipment, at cost:
Land		738,525		648,757
Buildings and improvements		3,000,208		2,618,265
Furniture and equipment		600,216		510,415
Property, plant and equipment, gross		9,120,588		8,361,315
Less: Accumulated depreciation		(2,632,909)		(2,384,033)
Total property, plant and equipment		6,487,679		5,977,282
Total assets		8,464,825		7,591,774
Liabilities:
Accounts payable and accrued expenses		461,518		441,667
Notes, loans and leases payable		3,493,908		3,262,880
Policy benefits and losses, claims and loss expenses payable		408,724		399,181
Liabilities from investment contracts
Other policyholders' funds and liabilities
Deferred income		27,491		28,696
Deferred income taxes, net		634,388		809,566
Related party liabilities		25,629		30,040
Total Liabilities		5,051,658		4,972,030
Stockholders' equity:
Additional paid-in capital		452,829		452,382
Accumulated other comprehensive loss		(9,911)		(51,236)
Retained earnings		3,643,043		2,892,683
Unearned employee stock ownership plan shares		(5,641)		(6,932)
Total stockholders' equity		3,413,167		2,619,744
Total liabilities and stockholders' equity		8,464,825		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		538,334		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		4,243,305		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		12,296	[1]	12,725	[2]	11,258	[3]	14,049	[3]
Reinsurance recoverables and trade receivables, net		100,849	[1]	107,757	[2]
Inventories, net			[1]		[2]
Prepaid expenses			[1]		[2]
Investments, fixed maturities and marketable equities		272,939	[1]	248,816	[2]
Investments, other		67,879	[1]	63,086	[2]
Deferred policy acquisition costs, net			[1]		[2]
Other assets		594	[1]	1,922	[2]
Related party assets		7,521	[1]	11,496	[2]
Subtotal assets		462,078	[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		462,078	[1]	445,802	[2]
Liabilities:
Accounts payable and accrued expenses		3,158	[1]	1,926	[2]
Notes, loans and leases payable			[1]		[2]
Policy benefits and losses, claims and loss expenses payable		238,085	[1]	244,980	[2]
Liabilities from investment contracts			[1]		[2]
Other policyholders' funds and liabilities		5,282	[1]	4,184	[2]
Deferred income			[1]		[2]
Deferred income taxes, net		13,510	[1]	11,243	[2]
Related party liabilities		2,341	[1]	2,539	[2]
Total Liabilities		262,376	[1]	264,872	[2]
Stockholders' equity:
Additional paid-in capital		91,120	[1]	91,120	[2]
Accumulated other comprehensive loss		12,301	[1]	6,166	[2]
Retained earnings		92,980	[1]	80,343	[2]
Unearned employee stock ownership plan shares			[1]		[2]
Total stockholders' equity		199,702	[1]	180,930	[2]
Total liabilities and stockholders' equity		462,078	[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		32,327	[1]	13,416	[2]	1,925	[3]	931	[3]
Reinsurance recoverables and trade receivables, net		30,760	[1]	29,090	[2]
Inventories, net			[1]		[2]
Prepaid expenses			[1]		[2]
Investments, fixed maturities and marketable equities		1,595,375	[1]	1,414,952	[2]
Investments, other		316,799	[1]	269,402	[2]
Deferred policy acquisition costs, net		124,820	[1]	130,213	[2]
Other assets		2,572	[1]	2,406	[2]
Related party assets		18,364	[1]	18,465	[2]
Subtotal assets		2,121,017	[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		2,121,017	[1]	1,877,944	[2]
Liabilities:
Accounts payable and accrued expenses		5,279	[1]	6,948	[2]
Notes, loans and leases payable			[1]		[2]
Policy benefits and losses, claims and loss expenses payable		443,397	[1]	442,161	[2]
Liabilities from investment contracts		1,319,945	[1]	1,112,498	[2]
Other policyholders' funds and liabilities		5,457	[1]	5,966	[2]
Deferred income			[1]		[2]
Deferred income taxes, net		23,650	[1]	14,200	[2]
Related party liabilities		117	[1]	43	[2]
Total Liabilities		1,797,845	[1]	1,581,816	[2]
Stockholders' equity:
Additional paid-in capital		26,271	[1]	26,271	[2]
Accumulated other comprehensive loss		30,616	[1]	16,933	[2]
Retained earnings		263,785	[1]	250,424	[2]
Unearned employee stock ownership plan shares			[1]		[2]
Total stockholders' equity		323,172	[1]	296,128	[2]
Total liabilities and stockholders' equity		2,121,017	[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]
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]	(28,087)		(32,622)
Subtotal assets		(28,087)		(32,622)
Investments in subsidiaries	[5]	(522,874)		(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		(550,961)		(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, net
Related party liabilities	[4]	(28,087)		(32,622)
Total Liabilities		(28,087)		(32,622)
Stockholders' equity:
Additional paid-in capital	[5]	(117,601)		(117,601)
Accumulated other comprehensive loss	[5]	(42,917)		(23,099)
Retained earnings	[5]	(356,555)		(330,557)
Unearned employee stock ownership plan shares
Total stockholders' equity		(522,874)		(477,058)
Total liabilities and stockholders' equity		(550,961)		(509,680)
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, 2017

[2] Balances as of December 31, 2016

[3] Balance for the period ended September 30, 2016

[4] Eliminate intercompany receivables and payables

[5] Eliminate investment in subsidiaries