Revenue Recognition - Schedule of Impact of Adoption on Condensed Consolidated Statement of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Sales:
Total sales		$ 535,027	$ 436,834
Cost and expenses:
Delivery and branch expenses		102,673	91,204
Operating income		6,063	31,066
Income before income taxes		3,288	28,670
Income tax expense		973	(1,512)
Net income		2,315	30,182
General Partner’s interest in net income		15	175
Limited Partners’ interest in net income		$ 2,300	$ 30,007
Basic and diluted income per Limited Partner Unit	[1]	$ 0.04	$ 0.45
Balances Without Adoption of ASC 606 | ASU 2014-09
Sales:
Total sales		$ 536,947
Cost and expenses:
Delivery and branch expenses		105,209
Operating income		5,447
Income before income taxes		2,672
Income tax expense		791
Net income		1,881
General Partner’s interest in net income		13
Limited Partners’ interest in net income		$ 1,868
Basic and diluted income per Limited Partner Unit		$ 0.04
Effect of Change Higher/(Lower) | ASU 2014-09
Sales:
Total sales		$ (1,920)
Cost and expenses:
Delivery and branch expenses		(2,536)
Operating income		616
Income before income taxes		616
Income tax expense		182
Net income		434
General Partner’s interest in net income		2
Limited Partners’ interest in net income		432
Product
Sales:
Total sales		458,707	$ 366,734
Product | Balances Without Adoption of ASC 606 | ASU 2014-09
Sales:
Total sales		462,153
Product | Effect of Change Higher/(Lower) | ASU 2014-09
Sales:
Total sales		(3,446)
Installations and services
Sales:
Total sales		76,320	$ 70,100
Installations and services | Balances Without Adoption of ASC 606 | ASU 2014-09
Sales:
Total sales		74,794
Installations and services | Effect of Change Higher/(Lower) | ASU 2014-09
Sales:
Total sales		$ 1,526

[1] See Note 15 - Earnings Per Limited Partner Unit.