Consolidating financial information by industry segment (Statements of Operations) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenues:
Self-moving equipment rentals	$ 804,325	$ 781,713	$ 1,552,921	$ 1,498,315
Self-storage revenues	104,965	91,493	203,239	177,705
Self moving and self-storage products and service sales	73,121	72,913	153,147	152,154
Property management fees	7,233	7,192	14,389	14,608
Life insurance premiums	32,355	35,920	65,065	72,808
Property and casualty insurance premiums	18,365	16,283	31,789	29,064
Net investment and interest income	33,098	28,227	68,847	52,832
Other revenue	76,752	70,766	140,066	126,598
Total revenues	1,150,214	1,104,507	2,229,463	2,124,084
Costs and expenses:
Operating expenses	565,413	529,350	1,099,885	1,025,904
Commission expenses	86,099	85,334	166,998	164,591
Cost of sales	43,930	46,402	92,859	96,283
Benefits and losses	45,825	45,773	94,831	94,327
Amortization of deferred policy acquisition costs	6,515	5,899	12,579	11,930
Lease expense	6,356	8,170	13,392	16,339
Depreciation, net of (gains) losses on disposals	151,553	132,625	292,153	259,052
Net (gains) losses on disposal of real estate	(217)	10	(1,839)	10
Total costs and expenses	905,474	853,563	1,770,858	1,668,436
Earnings (loss) from operations before equity in earnings of subsidiaries	244,740	250,944	458,605	455,648
Equity in earnings of subsidiaries	0	0	0	0
Earnings from operations	244,740	250,944	458,605	455,648
Pretax earnings	205,354	215,660	380,068	384,857
Earnings available to common stockholders	156,326	163,542	288,748	291,391
Operating Segments [Member] | Moving and Storage Consolidations [Member]
Revenues:
Self-moving equipment rentals	805,659	783,035	1,554,795	1,500,577
Self-storage revenues	104,965	91,493	203,239	177,705
Self moving and self-storage products and service sales	73,121	72,913	153,147	152,154
Property management fees	7,233	7,192	14,389	14,608
Life insurance premiums	0	0	0	0
Property and casualty insurance premiums	0	0	0	0
Net investment and interest income	3,031	2,830	6,298	5,393
Other revenue	75,543	69,471	138,082	124,382
Total revenues	1,069,552	1,026,934	2,069,950	1,974,819
Costs and expenses:
Operating expenses	552,708	518,383	1,075,232	1,002,003
Commission expenses	86,099	85,334	166,998	164,591
Cost of sales	43,930	46,402	92,859	96,283
Benefits and losses	0	0	0	0
Amortization of deferred policy acquisition costs	0	0	0	0
Lease expense	6,492	8,306	13,664	16,611
Depreciation, net of (gains) losses on disposals	151,553	132,625	292,153	259,052
Net (gains) losses on disposal of real estate	(217)	10	(1,839)	10
Total costs and expenses	840,565	791,060	1,639,067	1,538,550
Earnings (loss) from operations before equity in earnings of subsidiaries	228,987	235,874	430,883	436,269
Equity in earnings of subsidiaries	12,914	11,986	22,745	15,642
Earnings from operations	241,901	247,860	453,628	451,911
Other components of net periodic benefit costs	(264)	(254)	(527)	(507)
Interest expense	(39,402)	(35,321)	(78,568)	(70,860)
Pretax earnings	202,235	212,285	374,533	380,544
Income tax expense	(45,909)	(48,743)	(85,785)	(89,153)
Earnings available to common stockholders	156,326	163,542	288,748	291,391
Operating Segments [Member] | Property and Casualty Insurance [Member]
Revenues:
Self-moving equipment rentals	0	0	0	0
Self-storage revenues	0	0	0	0
Self moving and self-storage products and service sales	0	0	0	0
Property management fees	0	0	0	0
Life insurance premiums	0	0	0	0
Property and casualty insurance premiums	19,216	16,972	33,330	30,320
Net investment and interest income	4,312	3,550	10,503	6,091
Other revenue	0	0	0	0
Total revenues	23,528	20,522	43,833	36,411
Costs and expenses:
Operating expenses	9,395	7,915	17,476	16,615
Commission expenses	0	0	0	0
Cost of sales	0	0	0	0
Benefits and losses	5,041	4,748	8,799	9,224
Amortization of deferred policy acquisition costs	0	0	0	0
Lease expense	0	0	0	0
Depreciation, net of (gains) losses on disposals	0	0	0	0
Net (gains) losses on disposal of real estate	0	0	0	0
Total costs and expenses	14,436	12,663	26,275	25,839
Earnings (loss) from operations before equity in earnings of subsidiaries	9,092	7,859	17,558	10,572
Equity in earnings of subsidiaries	0	0	0	0
Earnings from operations	9,092	7,859	17,558	10,572
Other components of net periodic benefit costs	0	0	0	0
Interest expense	0	0	0	0
Pretax earnings	9,092	7,859	17,558	10,572
Income tax expense	(1,864)	(1,628)	(3,642)	(2,173)
Earnings available to common stockholders	7,228	6,231	13,916	8,399
Operating Segments [Member] | Life Insurance [Member]
Revenues:
Self-moving equipment rentals	0	0	0	0
Self-storage revenues	0	0	0	0
Self moving and self-storage products and service sales	0	0	0	0
Property management fees	0	0	0	0
Life insurance premiums	32,355	35,920	65,065	72,808
Property and casualty insurance premiums	0	0	0	0
Net investment and interest income	26,165	22,270	52,866	42,187
Other revenue	1,339	1,422	2,249	2,480
Total revenues	59,859	59,612	120,180	117,475
Costs and expenses:
Operating expenses	5,619	5,186	10,847	11,059
Commission expenses	0	0	0	0
Cost of sales	0	0	0	0
Benefits and losses	40,784	41,025	86,032	85,103
Amortization of deferred policy acquisition costs	6,515	5,899	12,579	11,930
Lease expense	0	0	0	0
Depreciation, net of (gains) losses on disposals	0	0	0	0
Net (gains) losses on disposal of real estate	0	0	0	0
Total costs and expenses	52,918	52,110	109,458	108,092
Earnings (loss) from operations before equity in earnings of subsidiaries	6,941	7,502	10,722	9,383
Equity in earnings of subsidiaries	0	0	0	0
Earnings from operations	6,941	7,502	10,722	9,383
Other components of net periodic benefit costs	0	0	0	0
Interest expense	0	0	0	0
Pretax earnings	6,941	7,502	10,722	9,383
Income tax expense	(1,255)	(1,747)	(1,893)	(2,140)
Earnings available to common stockholders	5,686	5,755	8,829	7,243
Consolidation, Eliminations [Member]
Revenues:
Self-moving equipment rentals	(1,334)	(1,322)	(1,874)	(2,262)
Self-storage revenues	0	0	0	0
Self moving and self-storage products and service sales	0	0	0	0
Property management fees	0	0	0	0
Life insurance premiums	0	0	0	0
Property and casualty insurance premiums	(851)	(689)	(1,541)	(1,256)
Net investment and interest income	(410)	(423)	(820)	(839)
Other revenue	(130)	(127)	(265)	(264)
Total revenues	(2,725)	(2,561)	(4,500)	(4,621)
Costs and expenses:
Operating expenses	(2,309)	(2,134)	(3,670)	(3,773)
Commission expenses	0	0	0	0
Cost of sales	0	0	0	0
Benefits and losses	0	0	0	0
Amortization of deferred policy acquisition costs	0	0	0	0
Lease expense	(136)	(136)	(272)	(272)
Depreciation, net of (gains) losses on disposals	0	0	0	0
Net (gains) losses on disposal of real estate	0	0	0	0
Total costs and expenses	(2,445)	(2,270)	(3,942)	(4,045)
Earnings (loss) from operations before equity in earnings of subsidiaries	(280)	(291)	(558)	(576)
Equity in earnings of subsidiaries	(12,914)	(11,986)	(22,745)	(15,642)
Earnings from operations	(13,194)	(12,277)	(23,303)	(16,218)
Other components of net periodic benefit costs	0	0	0	0
Interest expense	280	291	558	576
Pretax earnings	(12,914)	(11,986)	(22,745)	(15,642)
Income tax expense	0	0	0	0
Earnings available to common stockholders	$ (12,914)	$ (11,986)	$ (22,745)	$ (15,642)