Provision for Taxes (Narratives) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Components of Deferred Tax Assets and Liabilities [Abstract]
NOL and credit carryforwards, federal amount	$ 358.9
Federal statutory rate, blended rate for fiscal year	31.55%
Provisional benefit amount	$ 356.6
Deferred tax assets, effect of Tax Cuts and Jobs Act, tax expense (benefit)	371.5
Reclassified amount from accumulated other comprehensive income to retained earnings	8.7
Provisional tax expense for foreign tax credits	10.7
Phase three tax, increase in income tax expense	4.2
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued [Abstract]
Unrecognized tax benefits, income tax penalties and interest accrued	9.5	$ 8.5
Unrecognized tax benefits, income tax penalties and interest expense	1.0
Amerco [Member]
Components of Deferred Tax Assets and Liabilities [Abstract]
NOL and credit carryfowards, state amount	$ 172.3	$ 45.3