Revenue Recognition - Schedule of Impact of Adoption on Condensed Consolidated Statement of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended				6 Months Ended
		Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2019	Mar. 31, 2018
Sales:
Total sales		$ 699,582		$ 684,031		$ 1,234,609	$ 1,120,865
Cost and expenses:
Delivery and branch expenses		110,684		106,605		213,357	197,809
Operating income		105,002		85,473		111,065	116,539
Income before income taxes		101,564		82,783		104,852	111,453
Income tax expense		29,239		28,005		30,212	26,493
Net income		72,325	$ 2,315	54,778	$ 30,182	74,640	84,960
General Partner’s interest in net income		454		319		469	494
Limited Partners’ interest in net income		$ 71,871		$ 54,459		$ 74,171	$ 84,466
Basic and diluted income per Limited Partner Unit	[1]	$ 1.15		$ 0.81		$ 1.19	$ 1.26
Balances Without Adoption of ASC 606 | ASU 2014-09
Sales:
Total sales		$ 703,882				$ 1,240,829
Cost and expenses:
Delivery and branch expenses		111,163				216,372
Operating income		108,823				114,270
Income before income taxes		105,385				108,057
Income tax expense		30,344				31,135
Net income		75,041				76,922
General Partner’s interest in net income		470				483
Limited Partners’ interest in net income		$ 74,571				$ 76,439
Basic and diluted income per Limited Partner Unit		$ 1.19				$ 1.22
Effect of Change Higher/(Lower) | ASU 2014-09
Sales:
Total sales		$ (4,300)				$ (6,220)
Cost and expenses:
Delivery and branch expenses		(479)				(3,015)
Operating income		(3,821)				(3,205)
Income before income taxes		(3,821)				(3,205)
Income tax expense		(1,105)				(923)
Net income		(2,716)				(2,282)
General Partner’s interest in net income		(16)				(14)
Limited Partners’ interest in net income		$ (2,700)				$ (2,268)
Basic and diluted income per Limited Partner Unit		$ (0.04)				$ (0.03)
Product
Sales:
Total sales		$ 637,400		$ 622,962		$ 1,096,107	$ 989,696
Product | Balances Without Adoption of ASC 606 | ASU 2014-09
Sales:
Total sales		643,407				1,105,560
Product | Effect of Change Higher/(Lower) | ASU 2014-09
Sales:
Total sales		(6,007)				(9,453)
Installations and services
Sales:
Total sales		62,182		$ 61,069		138,502	$ 131,169
Installations and services | Balances Without Adoption of ASC 606 | ASU 2014-09
Sales:
Total sales		60,475				135,269
Installations and services | Effect of Change Higher/(Lower) | ASU 2014-09
Sales:
Total sales		$ 1,707				$ 3,233

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