Revenue Recognition - Schedule of Impact of Adoption on Condensed Consolidated Statement of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Sales:
Total sales		$ 283,376	$ 327,354	$ 1,517,985	$ 1,448,219
Cost and expenses:
Delivery and branch expenses		82,669	83,312	296,026	281,121
Operating income (loss)		(29,933)	(8,817)	81,132	107,722
Income (loss) before income taxes		(33,153)	(11,421)	71,699	100,032
Income tax expense (benefit)		(10,055)	(3,416)	20,157	23,077
Net income (loss)		(23,098)	(8,005)	51,542	76,955
General Partner’s interest in net income (loss)		(150)	(49)	319	445
Limited Partners’ interest in net income (loss)		$ (22,948)	$ (7,956)	$ 51,223	$ 76,510
Basic and diluted income (loss) per Limited Partner Unit	[1]	$ (0.46)	$ (0.15)	$ 0.86	$ 1.18
Balances Without Adoption of ASC 606 | ASU 2014-09
Sales:
Total sales		$ 283,224		$ 1,524,053
Cost and expenses:
Delivery and branch expenses		82,432		298,804
Operating income (loss)		(29,848)		84,422
Income (loss) before income taxes		(33,068)		74,989
Income tax expense (benefit)		(10,053)		21,082
Net income (loss)		(23,015)		53,907
General Partner’s interest in net income (loss)		(149)		334
Limited Partners’ interest in net income (loss)		$ (22,866)		$ 53,573
Basic and diluted income (loss) per Limited Partner Unit		$ (0.46)		$ 0.90
Effect of Change Higher/(Lower) | ASU 2014-09
Sales:
Total sales		$ 152		$ (6,068)
Cost and expenses:
Delivery and branch expenses		237		(2,778)
Operating income (loss)		(85)		(3,290)
Income (loss) before income taxes		(85)		(3,290)
Income tax expense (benefit)		(2)		(925)
Net income (loss)		(83)		(2,365)
General Partner’s interest in net income (loss)		(1)		(15)
Limited Partners’ interest in net income (loss)		(82)		$ (2,350)
Basic and diluted income (loss) per Limited Partner Unit				$ (0.04)
Product
Sales:
Total sales		210,657	$ 256,447	$ 1,306,764	$ 1,246,143
Product | Balances Without Adoption of ASC 606 | ASU 2014-09
Sales:
Total sales		212,249		1,317,809
Product | Effect of Change Higher/(Lower) | ASU 2014-09
Sales:
Total sales		(1,592)		(11,045)
Installations and services
Sales:
Total sales		72,719	$ 70,907	211,221	$ 202,076
Installations and services | Balances Without Adoption of ASC 606 | ASU 2014-09
Sales:
Total sales		70,975		206,244
Installations and services | Effect of Change Higher/(Lower) | ASU 2014-09
Sales:
Total sales		$ 1,744		$ 4,977

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