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Borrowings (Narratives) (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
Borrowings (Narratives) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2017
Debt instruments, face, payment, and remaining balance amount:
Notes, loans and leases payable	$ 3,493,908			$ 3,493,908		$ 3,262,880
Interest paid related to derivative contracts [Abstract]
Interest paid in cash	32,700		$ 28,700	96,100	$ 84,000
Real estate loan [Member]
Debt instruments, face, payment, and remaining balance amount:
Notes, loans and leases payable	137,786			137,786		169,289
Senior Mortgages [Member]
Debt instruments, face, payment, and remaining balance amount:
Notes, loans and leases payable	1,472,209			1,472,209		1,292,160
Working Capital Loans [Member]
Debt instruments, face, payment, and remaining balance amount:
Notes, loans and leases payable	55,000			$ 55,000		85,000
Working Capital Loans Two [Member] \| AMERCO [Member]
Debt instruments, interest rate, effective percentage:
Unused capacity fee				0.30%
Debt instruments, face, payment, and remaining balance amount:
Debt instrument, original face amount	150,000			$ 150,000
Line of credit facility, maximum borrowing capacity	300,000			300,000
Line of credit, current borrowing capacity	$ 150,000			$ 150,000
Debt instruments, issuance and maturity dates:
Debt instrument, maturity date				Sep. 30, 2020
Working Capital Loans Two [Member] \| AMERCO [Member] \| Minimum [Member]
Debt instruments, interest rate, effective percentage:
LIBOR	1.38%			1.38%
Working Capital Loans Two [Member] \| AMERCO [Member] \| Maximum [Member]
Debt instruments, interest rate, effective percentage:
LIBOR	1.50%			1.50%
Rental Truck (amortizing loans) First Loan [Member]
Debt instruments, face, payment, and remaining balance amount:
Notes, loans and leases payable	$ 322,542			$ 322,542		324,977
Rental Truck (securitizations) [Member]
Debt instruments, face, payment, and remaining balance amount:
Notes, loans and leases payable						52,112
Capital Lease Obligations [Member]
Debt instruments, face, payment, and remaining balance amount:
Notes, loans and leases payable	982,653			982,653		876,828
Other Obligations [Member]
Debt instruments, face, payment, and remaining balance amount:
Notes, loans and leases payable	72,788			72,788		69,867
Other Obligations [Member] \| AMERCO [Member]
Debt instruments, face, payment, and remaining balance amount:
Notes, loans and leases payable	76,700			76,700
Subsidiary holdings of parent company debt	$ 3,900			$ 3,900
Debt instruments, issuance and maturity dates:
Debt instrument, maturity date range, start				Jan. 01, 2018
Debt instrument, maturity date range, end				Dec. 31, 2047
Other Obligations [Member] \| AMERCO [Member] \| Minimum [Member]
Debt instruments, interest rate, stated percentage:
Debt instrument, interest rate, stated percentage	2.75%			2.75%
Other Obligations [Member] \| AMERCO [Member] \| Maximum [Member]
Debt instruments, interest rate, stated percentage:
Debt instrument, interest rate, stated percentage	8.00%			8.00%
Amerco Real Estate Subsidiaries and Uhaul Company of Florida [Member] \| Real estate loan [Member] \| AMERCO [Member]
Debt instruments, interest rate, stated percentage:
Debt instrument, interest rate, stated percentage	6.93%			6.93%
Debt instruments, interest rate, effective percentage:
LIBOR	1.44%			1.44%
Applicable margin interest rate	1.50%			1.50%
Sum of LIBOR and margin	2.94%			2.94%
Debt instrument, interest rate at period end				1.50%
Debt instruments, face, payment, and remaining balance amount:
Line of credit facility, remaining borrowing capacity	$ 62,200			$ 62,200
Amounts held at LIBOR plus margin	75,600			75,600
Notes, loans and leases payable	137,800			$ 137,800
Debt instruments, issuance and maturity dates:
Mortgage loans on real estate, final maturity date				Apr. 30, 2023
Debt instrument, maturity date				Aug. 31, 2018
Various Subsidiaries of Amerco Real Estate and Uhaul Intl [Member] \| Senior Mortgages [Member]
Debt instruments, face, payment, and remaining balance amount:
Notes, loans and leases payable	$ 1,472,200			$ 1,472,200
Debt instruments, issuance and maturity dates:
Debt instrument, maturity date range, start				Jan. 01, 2021
Debt instrument, maturity date range, end				Dec. 31, 2038
Various Subsidiaries of Amerco Real Estate and Uhaul Intl [Member] \| Senior Mortgages [Member] \| Minimum [Member]
Debt instruments, interest rate, stated percentage:
Debt instrument, interest rate, stated percentage	3.72%			3.72%
Various Subsidiaries of Amerco Real Estate and Uhaul Intl [Member] \| Senior Mortgages [Member] \| Maximum [Member]
Debt instruments, interest rate, stated percentage:
Debt instrument, interest rate, stated percentage	6.62%			6.62%
Various Subsidiaries of Amerco Real Estate and Uhaul Intl [Member] \| Working Capital Loans [Member] \| AMERCO [Member]
Debt instruments, interest rate, effective percentage:
LIBOR	1.33%			1.33%
Applicable margin interest rate	1.50%			1.50%
Sum of LIBOR and margin	2.83%			2.83%
Debt instruments, face, payment, and remaining balance amount:
Line of credit facility, maximum borrowing capacity	$ 55,000			$ 55,000
Notes, loans and leases payable	55,000			$ 55,000
Debt instruments, issuance and maturity dates:
Debt instrument, maturity date				Nov. 30, 2018
Uhaul Intl and Subsidiaries [Member] \| Rental Truck (amortizing loans) First Loan [Member] \| AMERCO [Member]
Debt instruments, face, payment, and remaining balance amount:
Notes, loans and leases payable	$ 322,500			$ 322,500
Debt instruments, issuance and maturity dates:
Debt instrument, maturity date range, start				Jun. 01, 2018
Debt instrument, maturity date range, end				Nov. 30, 2024
Uhaul Intl and Subsidiaries [Member] \| Rental Truck (amortizing loans) First Loan [Member] \| AMERCO [Member] \| Minimum [Member]
Debt instruments, interest rate, stated percentage:
Debt instrument, interest rate, stated percentage	2.82%			2.82%
Debt instruments, interest rate, effective percentage:
LIBOR	1.36%			1.36%
Applicable margin interest rate	1.72%			1.72%
Uhaul Intl and Subsidiaries [Member] \| Rental Truck (amortizing loans) First Loan [Member] \| AMERCO [Member] \| Maximum [Member]
Debt instruments, interest rate, stated percentage:
Debt instrument, interest rate, stated percentage	4.76%			4.76%
Debt instruments, interest rate, effective percentage:
LIBOR	1.48%			1.48%
Applicable margin interest rate	2.38%			2.38%
Uhaul Intl and Subsidiaries [Member] \| Rental Truck (amortizing loans) Second Loan [Member] \| AMERCO [Member]
Debt instruments, face, payment, and remaining balance amount:
Notes, loans and leases payable	$ 265,700			$ 265,700
Uhaul Intl and Subsidiaries [Member] \| Rental Truck (amortizing loans) Second Loan [Member] \| AMERCO [Member] \| Minimum [Member]
Debt instruments, interest rate, stated percentage:
Debt instrument, interest rate, stated percentage	1.95%			1.95%
Uhaul Intl and Subsidiaries [Member] \| Rental Truck (amortizing loans) Second Loan [Member] \| AMERCO [Member] \| Maximum [Member]
Debt instruments, interest rate, stated percentage:
Debt instrument, interest rate, stated percentage	3.94%			3.94%
Uhaul Intl and Subsidiaries [Member] \| Revolving fleet loans [Member]
Debt instruments, interest rate, effective percentage:
Applicable margin interest rate	1.15%			1.15%
Debt instruments, face, payment, and remaining balance amount:
Debt instrument, original face amount	$ 490,000			$ 490,000
Line of credit facility, maximum borrowing capacity	565,000			565,000
Notes, loans and leases payable	$ 477,000			$ 477,000
Debt instruments, issuance and maturity dates:
Debt instrument, maturity date range, start				Mar. 01, 2020
Debt instrument, maturity date range, end				Nov. 30, 2021
Uhaul Intl and Subsidiaries [Member] \| Revolving fleet loans [Member] \| Minimum [Member]
Debt instruments, interest rate, effective percentage:
LIBOR	1.35%			1.35%
Sum of LIBOR and margin	2.50%			2.50%
Uhaul Intl and Subsidiaries [Member] \| Revolving fleet loans [Member] \| Maximum [Member]
Debt instruments, interest rate, effective percentage:
LIBOR	1.36%			1.36%
Sum of LIBOR and margin	2.51%			2.51%
Uhaul S Fleet Two Thousand Ten Box Truck Note [Member] \| Rental Truck (securitizations) [Member]
Debt instruments, interest rate, stated percentage:
Debt instrument, interest rate, stated percentage	4.90%			4.90%
Debt instruments, face, payment, and remaining balance amount:
Debt instrument, original face amount	$ 155,000			$ 155,000
Amerco, Us Bank, National Association, Trustee [Member] \| Capital Lease Obligations [Member] \| AMERCO [Member]
Debt instruments, face, payment, and remaining balance amount:
New capital lease obligations, amount				323,700
Notes, loans and leases payable	982,700			982,700		876,800
Capitalized assets, net book value	$ 1,382,500			$ 1,382,500		$ 1,233,300
Life Insurance [Member] \| Federal Home Loan Bank [Member]
Debt instruments, issuance and maturity dates:
Debt instrument, maturity date range, start		Mar. 29, 2018
Debt instrument, maturity date range, end		Mar. 30, 2022
Federal home loan bank activity for current period:
Aggregate deposit amount		$ 60,000
Available for sale investments, noncurrent		129,600
Amount pledged as collateral		$ 73,400
Life Insurance [Member] \| Federal Home Loan Bank [Member] \| Minimum [Member]
Federal home loan bank activity for current period:
Aggregate deposit amount fixed interest rate		1.33%
Life Insurance [Member] \| Federal Home Loan Bank [Member] \| Maximum [Member]
Federal home loan bank activity for current period:
Aggregate deposit amount fixed interest rate		1.75%