Supplemental Guarantor Information - Condensed Consolidating Statements of Operations (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Net sales		$ 1,429,037	$ 1,425,625	$ 3,150,713	$ 2,547,604
Cost of products sold		1,094,049	1,087,248	2,380,156	1,939,474
Gross profit		334,988	338,377	770,557	608,130
Operating expense:
Selling, general and administrative		320,408	341,587	648,101	535,340
Depreciation		17,447	17,120	35,048	25,829
Amortization		51,763	37,068	103,784	55,263
Total operating expense		389,618	395,775	786,933	616,432
Intercompany charges (income)		0	0	0	0
Income (loss) from operations		(54,630)	(57,398)	(16,376)	(8,302)
Interest expense, financing costs, and other		40,452	39,570	78,813	62,138
Intercompany interest expense (income)		0	0	0	0
Income (loss) before provision for income taxes		(95,082)	(96,968)	(95,189)	(70,440)
Provision for (benefit from) income taxes	[1]	(26,996)	(30,313)	(26,210)	(71,381)
Income (loss) before equity in net income of subsidiaries		(68,086)	(66,655)	(68,979)	941
Equity in net income of subsidiaries		0	0	0	0
Net income (loss)		(68,086)	(66,655)	(68,979)	941
Reportable Legal Entities [Member] | Parent Company [Member]
Net sales		0	0	0	0
Cost of products sold		0	0	0	0
Gross profit		0	0	0	0
Operating expense:
Selling, general and administrative		4,420	2,672	14,195	2,539
Depreciation		736	443	1,500	899
Amortization		0	0	0	0
Total operating expense		5,156	3,115	15,695	3,438
Intercompany charges (income)		(6,857)	(1,477)	(13,544)	(584)
Income (loss) from operations		1,701	(1,638)	(2,151)	(2,854)
Interest expense, financing costs, and other		27,929	47,220	62,242	57,296
Intercompany interest expense (income)		(1,209)	(5,181)	(10,889)	(10,889)
Income (loss) before provision for income taxes		(25,019)	(43,677)	(53,504)	(49,261)
Provision for (benefit from) income taxes		(7,685)	(10,469)	(15,039)	(4,948)
Income (loss) before equity in net income of subsidiaries		(17,334)	(33,208)	(38,465)	(44,313)
Equity in net income of subsidiaries		(50,752)	(33,447)	(30,514)	45,254
Net income (loss)		(68,086)	(66,655)	(68,979)	941
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Net sales		1,406,402	1,404,617	3,085,297	2,480,879
Cost of products sold		1,075,918	1,070,605	2,328,213	1,887,041
Gross profit		330,484	334,012	757,084	593,838
Operating expense:
Selling, general and administrative		308,830	331,072	618,378	515,953
Depreciation		16,242	16,238	32,614	24,048
Amortization		51,653	36,935	103,564	54,999
Total operating expense		376,725	384,245	754,556	595,000
Intercompany charges (income)		6,857	1,477	13,544	584
Income (loss) from operations		(53,098)	(51,710)	(11,016)	(1,746)
Interest expense, financing costs, and other		12,475	3,807	15,960	4,632
Intercompany interest expense (income)		827	4,804	10,125	10,125
Income (loss) before provision for income taxes		(66,400)	(60,321)	(37,101)	(16,503)
Provision for (benefit from) income taxes		(18,276)	(18,575)	(9,876)	(65,247)
Income (loss) before equity in net income of subsidiaries		(48,124)	(41,746)	(27,225)	48,744
Equity in net income of subsidiaries		0	0	0	0
Net income (loss)		(48,124)	(41,746)	(27,225)	48,744
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Net sales		22,635	21,008	65,416	66,725
Cost of products sold		18,131	16,643	51,943	52,433
Gross profit		4,504	4,365	13,473	14,292
Operating expense:
Selling, general and administrative		7,158	7,843	15,528	16,848
Depreciation		469	439	934	882
Amortization		110	133	220	264
Total operating expense		7,737	8,415	16,682	17,994
Intercompany charges (income)		0	0	0	0
Income (loss) from operations		(3,233)	(4,050)	(3,209)	(3,702)
Interest expense, financing costs, and other		48	(11,457)	611	210
Intercompany interest expense (income)		382	377	764	764
Income (loss) before provision for income taxes		(3,663)	7,030	(4,584)	(4,676)
Provision for (benefit from) income taxes		(1,035)	(1,269)	(1,295)	(1,186)
Income (loss) before equity in net income of subsidiaries		(2,628)	8,299	(3,289)	(3,490)
Equity in net income of subsidiaries		0	0	0	0
Net income (loss)		(2,628)	8,299	(3,289)	(3,490)
Eliminations and Other [Member]
Net sales		0	0	0	0
Cost of products sold		0	0	0	0
Gross profit		0	0	0	0
Operating expense:
Selling, general and administrative		0	0	0	0
Depreciation		0	0	0	0
Amortization		0	0	0	0
Total operating expense		0	0	0	0
Intercompany charges (income)		0	0	0	0
Income (loss) from operations		0	0	0	0
Interest expense, financing costs, and other		0	0	0	0
Intercompany interest expense (income)		0	0	0	0
Income (loss) before provision for income taxes		0	0	0	0
Provision for (benefit from) income taxes		0	0	0	0
Income (loss) before equity in net income of subsidiaries		0	0	0	0
Equity in net income of subsidiaries		50,752	33,447	30,514	(45,254)
Net income (loss)		$ 50,752	$ 33,447	$ 30,514	$ (45,254)

[1] Three and six months ended March 31, 2018 amounts include a non-recurring net tax benefit of $1.5 million and $48.0 million, respectively, resulting from recording the provisional impact of the enactment of the 2017 Tax Cuts and Jobs Act (“TCJA”).