Segment Reporting - Summarized Financial Information for Company's Reportable Segments (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Rental revenues		$ 82,696	$ 74,261
Rental related services revenues		21,455	17,831
Sales and other revenues		17,857	12,993
Total revenues		122,008	105,085
Depreciation of rental equipment		18,961	17,777
Gross profit		57,005	50,170
Selling and administrative expenses		29,695	28,128
Income from operations		27,310	22,042
Interest (expense) income allocation		(3,108)	(2,992)
Income (loss) before provision for income taxes		24,251	19,018
Rental equipment acquisitions		35,440	24,692
Accounts receivable, net (period end)		119,403		$ 121,016		$ 98,092
Rental equipment, at cost (period end)		1,436,251		1,416,000		1,359,360
Rental equipment, net book value (period end)		912,878		$ 901,015	$ 912,878	865,338
Mobile Modular [Member]
Segment Reporting Information [Line Items]
Rental revenues		42,261	37,027
Rental related services revenues		14,471	11,934
Sales and other revenues		8,360	4,890
Total revenues		65,092	53,851
Depreciation of rental equipment		5,408	5,248
Gross profit		30,879	26,321
Selling and administrative expenses		15,370	14,012
Income from operations		15,509	12,309
Interest (expense) income allocation		(1,883)	(1,676)
Income (loss) before provision for income taxes		13,626	10,633
Rental equipment acquisitions		20,938	$ 8,233
Accounts receivable, net (period end)		71,816				55,883
Rental equipment, at cost (period end)		$ 834,883				779,839
Rental equipment, net book value (period end)					585,248	544,707
Utilization (period end)	[1]	78.90%	76.90%
Average utilization	[1]	78.80%	77.30%
TRS-RenTelco [Member]
Segment Reporting Information [Line Items]
Rental revenues		$ 23,623	$ 21,529
Rental related services revenues		708	807
Sales and other revenues		6,345	5,702
Total revenues		30,676	28,038
Depreciation of rental equipment		9,520	8,577
Gross profit		13,667	12,848
Selling and administrative expenses		5,970	5,618
Income from operations		7,697	7,230
Interest (expense) income allocation		(678)	(683)
Income (loss) before provision for income taxes		7,068	6,515
Rental equipment acquisitions		12,808	$ 15,532
Accounts receivable, net (period end)		20,925				19,902
Rental equipment, at cost (period end)		$ 286,469				269,043
Rental equipment, net book value (period end)					132,607	114,799
Utilization (period end)	[1]	66.50%	63.50%
Average utilization	[1]	64.30%	62.70%
Adler Tanks [Member]
Segment Reporting Information [Line Items]
Rental revenues		$ 16,812	$ 15,705
Rental related services revenues		6,276	5,090
Sales and other revenues		347	383
Total revenues		23,435	21,178
Depreciation of rental equipment		4,033	3,952
Gross profit		11,423	10,396
Selling and administrative expenses		7,080	7,198
Income from operations		4,343	3,198
Interest (expense) income allocation		(827)	(775)
Income (loss) before provision for income taxes		3,516	2,423
Rental equipment acquisitions		1,694	$ 927
Accounts receivable, net (period end)		19,151				16,490
Rental equipment, at cost (period end)		$ 314,899				310,478
Rental equipment, net book value (period end)					$ 195,023	205,832
Utilization (period end)	[1]	59.20%	57.50%
Average utilization	[1]	57.30%	57.60%
Enviroplex [Member]
Segment Reporting Information [Line Items]
Sales and other revenues	[2]	$ 2,805	$ 2,018
Total revenues	[2]	2,805	2,018
Gross profit	[2]	1,036	605
Selling and administrative expenses	[2]	1,275	1,300
Income from operations	[2]	(239)	(695)
Interest (expense) income allocation	[2]	280	142
Income (loss) before provision for income taxes	[2]	41	$ (553)
Accounts receivable, net (period end)	[2]	$ 7,511				$ 5,817

[1] Utilization is calculated each month by dividing the cost of rental equipment on rent by the total cost of rental equipment excluding accessory equipment and for Mobile Modular and Adler Tanks excluding new equipment inventory. The Average utilization for the period is calculated using the average costs of rental equipment.

[2] Gross Enviroplex sales revenues were $2,805 and $2,813 for the three months ended March 31, 2019 and 2018, respectively. There were $795 inter-segment sales to Mobile Modular in the three months ended March 31, 2018, which required elimination in consolidation. There were no inter-segment sales in the three months ended March 31, 2019.