Consolidating financial information by industry segment (Statement of Operations) (Details) - USD ($) $ in Thousands		12 Months Ended
		Mar. 31, 2018		Mar. 31, 2017		Mar. 31, 2016
Revenues:
Self-moving equipment rentals		$ 2,479,742		$ 2,362,833		$ 2,297,980
Self-storage revenues		323,903		286,886		247,944
Self-moving and self-storage products and service sales		261,557		253,073		251,541
Property management fees		29,602		29,075		26,533
Life insurance premiums		154,703		163,579		162,662
Property and casualty insurance premiums		57,100		52,334		50,020
Net investment and interest income		110,473		102,276		86,617
Other revenue		184,034		171,711		152,171
Total revenues		3,601,114		3,421,767		3,275,468
Costs and expenses:
Operating expenses		1,807,983		1,568,083		1,470,047
Commission expenses		276,705		267,230		262,627
Cost of sales		160,489		152,485		144,990
Benefits and losses		185,311		182,710		167,436
Amortization of deferred policy acquisition costs		24,514		26,218		23,272
Lease expense		33,960		37,343		49,780
Depreciation, net of (gains) losses on disposals		543,247		449,025		291,235
Net gains on disposal of real estate		(195,414)		(3,590)		(545)
Total costs and expenses		2,836,795		2,679,504		2,408,842
Earnings from operations before equity in earnings of subsidiaries		764,319		742,263		866,626
Equity in earnings of subsidiaries
Earnings from operations		764,319		742,263		866,626
Interest expense		(126,706)		(113,406)		(97,715)
Fees and amortization on early extinguishment of debt				(499)
Pretax earnings		637,613		628,358		768,911
Income tax expense		152,970		(229,934)		(279,910)
Earnings available to common shareholders		790,583		398,424		489,001
Operating Segments [Member] | Moving and Storage Consolidations [Member]
Revenues:
Self-moving equipment rentals		2,483,956		2,366,526		2,301,586
Self-storage revenues		323,903		286,886		247,944
Self-moving and self-storage products and service sales		261,557		253,073		251,541
Property management fees		29,602		29,075		26,533
Life insurance premiums
Property and casualty insurance premiums
Net investment and interest income		12,232		9,688		8,801
Other revenue		179,417		167,752		148,099
Total revenues		3,290,667		3,113,000		2,984,504
Costs and expenses:
Operating expenses		1,759,624		1,521,408		1,423,107
Commission expenses		276,705		267,230		262,627
Cost of sales		160,489		152,485		144,990
Benefits and losses
Amortization of deferred policy acquisition costs
Lease expense		34,243		37,529		49,966
Depreciation, net of (gains) losses on disposals		543,247		449,025		291,235
Net gains on disposal of real estate		(195,414)		(3,590)		(545)
Total costs and expenses		2,578,894		2,424,087		2,171,380
Earnings from operations before equity in earnings of subsidiaries		711,773		688,913		813,124
Equity in earnings of subsidiaries		46,990		35,797		35,522
Earnings from operations		758,763		724,710		848,646
Interest expense		(127,997)		(114,863)		(98,533)
Fees and amortization on early extinguishment of debt				(499)
Pretax earnings		630,766		609,348		750,113
Income tax expense		159,817		(210,924)		(261,112)
Earnings available to common shareholders		790,583		398,424		489,001
Operating Segments [Member] | Property and Casualty Insurance [Member]
Revenues:
Self-moving equipment rentals			[1]		[2]		[3]
Self-storage revenues			[1]		[2]		[3]
Self-moving and self-storage products and service sales			[1]		[2]		[3]
Property management fees			[1]		[2]		[3]
Life insurance premiums			[1]		[2]		[3]
Property and casualty insurance premiums		58,800	[1]	52,334	[2]	50,020	[3]
Net investment and interest income		15,771	[1]	16,652	[2]	14,783	[3]
Other revenue			[1]		[2]		[3]
Total revenues		74,571	[1]	68,986	[2]	64,803	[3]
Costs and expenses:
Operating expenses		32,710	[1]	28,421	[2]	27,958	[3]
Commission expenses			[1]		[2]		[3]
Cost of sales			[1]		[2]		[3]
Benefits and losses		15,983	[1]	13,404	[2]	12,298	[3]
Amortization of deferred policy acquisition costs			[1]		[2]		[3]
Lease expense			[1]		[2]		[3]
Depreciation, net of (gains) losses on disposals			[1]		[2]		[3]
Net gains on disposal of real estate
Total costs and expenses		48,693	[1]	41,825	[2]	40,256	[3]
Earnings from operations before equity in earnings of subsidiaries		25,878	[1]	27,161	[2]	24,547	[3]
Equity in earnings of subsidiaries			[1]		[2]		[3]
Earnings from operations		25,878	[1]	27,161	[2]	24,547	[3]
Interest expense			[1]		[2]		[3]
Fees and amortization on early extinguishment of debt
Pretax earnings		25,878	[1]	27,161	[2]	24,547	[3]
Income tax expense		(2,989)	[1]	(9,346)	[2]	(8,379)	[3]
Earnings available to common shareholders		22,889	[1]	17,815	[2]	16,168	[3]
Operating Segments [Member] | Life Insurance [Member]
Revenues:
Self-moving equipment rentals			[1]		[2]		[3]
Self-storage revenues			[1]		[2]		[3]
Self-moving and self-storage products and service sales			[1]		[2]		[3]
Property management fees			[1]		[2]		[3]
Life insurance premiums		154,703	[1]	163,579	[2]	162,662	[3]
Property and casualty insurance premiums			[1]		[2]		[3]
Net investment and interest income		84,158	[1]	77,540	[2]	63,999	[3]
Other revenue		5,001	[1]	4,480	[2]	4,559	[3]
Total revenues		243,862	[1]	245,599	[2]	231,220	[3]
Costs and expenses:
Operating expenses		22,061	[1]	22,429	[2]	23,037	[3]
Commission expenses			[1]		[2]		[3]
Cost of sales			[1]		[2]		[3]
Benefits and losses		169,328	[1]	169,306	[2]	155,138	[3]
Amortization of deferred policy acquisition costs		24,514	[1]	26,218	[2]	23,272	[3]
Lease expense			[1]		[2]		[3]
Depreciation, net of (gains) losses on disposals			[1]		[2]		[3]
Net gains on disposal of real estate
Total costs and expenses		215,903	[1]	217,953	[2]	201,447	[3]
Earnings from operations before equity in earnings of subsidiaries		27,959	[1]	27,646	[2]	29,773	[3]
Equity in earnings of subsidiaries			[1]		[2]		[3]
Earnings from operations		27,959	[1]	27,646	[2]	29,773	[3]
Interest expense			[1]		[2]		[3]
Fees and amortization on early extinguishment of debt
Pretax earnings		27,959	[1]	27,646	[2]	29,773	[3]
Income tax expense		(3,858)	[1]	(9,664)	[2]	(10,419)	[3]
Earnings available to common shareholders		24,101	[1]	17,982	[2]	19,354	[3]
Eliminations [Member]
Revenues:
Self-moving equipment rentals	[4]	(4,214)		(3,693)		(3,606)
Self-storage revenues
Self-moving and self-storage products and service sales
Property management fees
Life insurance premiums
Property and casualty insurance premiums		(1,700)
Net investment and interest income	[5]	(1,688)		(1,604)		(966)
Other revenue	[5]	(384)		(521)		(487)
Total revenues		(7,986)		(5,818)		(5,059)
Costs and expenses:
Operating expenses	[4],[5]	(6,412)		(4,175)		(4,055)
Commission expenses
Cost of sales
Benefits and losses
Amortization of deferred policy acquisition costs
Lease expense	[5]	(283)		(186)		(186)
Depreciation, net of (gains) losses on disposals
Net gains on disposal of real estate
Total costs and expenses		(6,695)		(4,361)		(4,241)
Earnings from operations before equity in earnings of subsidiaries		(1,291)		(1,457)		(818)
Equity in earnings of subsidiaries	[6]	(46,990)		(35,797)		(35,522)
Earnings from operations		(48,281)		(37,254)		(36,340)
Interest expense	[5]	1,291		1,457		818
Fees and amortization on early extinguishment of debt
Pretax earnings		(46,990)		(35,797)		(35,522)
Income tax expense
Earnings available to common shareholders		$ (46,990)		$ (35,797)		$ (35,522)

[1] Balances for the year ended December 31, 2016

[2] Balances for the year ended December 31, 2015

[3] Balances for the year ended December 31, 2014

[4] Eliminate intercompany premiums

[5] Eliminate intercompany lease / interest income

[6] Eliminate equity in earnings of subsidiaries