Supplemental Guarantor Information - Condensed Consolidating Statements of Operations (Detail) - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Net sales		$ 7,105,160	$ 6,418,311	$ 4,376,670
Cost of products sold		5,368,605	4,824,990	3,300,731
Gross profit		1,736,555	1,593,321	1,075,939
Operating expense
Selling, general and administrative		1,311,043	1,187,192	743,376
Depreciation		70,695	60,318	34,002
Amortization		207,065	141,185	82,465
Total operating expense		1,588,803	1,388,695	859,843
Intercompany charges (income)		0	0	0
Income (loss) from operations		147,752	204,626	216,096
Interest expense, financing costs, and other		158,534	136,544	52,751
Intercompany interest expense (income)		0	0	0
Income (loss) before provision for income taxes		(10,782)	68,082	163,345
Provision for (benefit from) income taxes	[1]	(170)	(30,544)	62,481
Income (loss) before equity in net income of subsidiaries		(10,612)	98,626	100,864
Equity in net income of subsidiaries		0	0	0
Net income (loss)		(10,612)	98,626	100,864
Reportable Legal Entities [Member] | Parent Company [Member]
Net sales		0	0	0
Cost of products sold		0	0	0
Gross profit		0	0	0
Operating expense
Selling, general and administrative		17,627	5,909	26,962
Depreciation		4,027	1,989	1,534
Amortization		0	0	0
Total operating expense		21,654	7,898	28,496
Intercompany charges (income)		(17,576)	(6,734)	(4,664)
Income (loss) from operations		(4,078)	(1,164)	(23,832)
Interest expense, financing costs, and other		140,489	124,169	38,660
Intercompany interest expense (income)		(39,670)	(24,270)	(24,458)
Income (loss) before provision for income taxes		(104,897)	(101,063)	(38,034)
Provision for (benefit from) income taxes		(116)	11,398	(3,284)
Income (loss) before equity in net income of subsidiaries		(104,781)	(112,461)	(34,750)
Equity in net income of subsidiaries		94,169	211,087	135,614
Net income (loss)		(10,612)	98,626	100,864
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Net sales		6,918,518	6,239,332	4,198,935
Cost of products sold		5,220,994	4,685,616	3,162,896
Gross profit		1,697,524	1,553,716	1,036,039
Operating expense
Selling, general and administrative		1,261,370	1,147,390	685,690
Depreciation		64,801	56,598	30,758
Amortization		206,627	140,664	81,880
Total operating expense		1,532,798	1,344,652	798,328
Intercompany charges (income)		17,132	6,565	4,342
Income (loss) from operations		147,594	202,499	233,369
Interest expense, financing costs, and other		17,327	11,648	14,033
Intercompany interest expense (income)		38,142	22,738	22,927
Income (loss) before provision for income taxes		92,125	168,113	196,409
Provision for (benefit from) income taxes		(632)	(42,261)	64,399
Income (loss) before equity in net income of subsidiaries		92,757	210,374	132,010
Equity in net income of subsidiaries		0	0	0
Net income (loss)		92,757	210,374	132,010
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Net sales		186,642	178,979	177,735
Cost of products sold		147,611	139,374	137,835
Gross profit		39,031	39,605	39,900
Operating expense
Selling, general and administrative		32,046	33,893	30,724
Depreciation		1,867	1,731	1,710
Amortization		438	521	585
Total operating expense		34,351	36,145	33,019
Intercompany charges (income)		444	169	322
Income (loss) from operations		4,236	3,291	6,559
Interest expense, financing costs, and other		718	727	58
Intercompany interest expense (income)		1,528	1,532	1,531
Income (loss) before provision for income taxes		1,990	1,032	4,970
Provision for (benefit from) income taxes		578	319	1,366
Income (loss) before equity in net income of subsidiaries		1,412	713	3,604
Equity in net income of subsidiaries		0	0	0
Net income (loss)		1,412	713	3,604
Eliminations and Other [Member]
Net sales		0	0	0
Cost of products sold		0	0	0
Gross profit		0	0	0
Operating expense
Selling, general and administrative		0	0	0
Depreciation		0	0	0
Amortization		0	0	0
Total operating expense		0	0	0
Intercompany charges (income)		0	0	0
Income (loss) from operations		0	0	0
Interest expense, financing costs, and other		0	0	0
Intercompany interest expense (income)		0	0	0
Income (loss) before provision for income taxes		0	0	0
Provision for (benefit from) income taxes		0	0	0
Income (loss) before equity in net income of subsidiaries		0	0	0
Equity in net income of subsidiaries		(94,169)	(211,087)	(135,614)
Net income (loss)		$ (94,169)	$ (211,087)	$ (135,614)

[1] Year ended September 30, 2018 amounts include a net tax benefit $49.2 million resulting from recording the provisional impact of the enactment of the 2017 Tax Cuts and Jobs Act (“TCJA”).