Supplemental Guarantor Information - Condensed Consolidating Balance Sheets (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Current assets:
Cash and cash equivalents	$ 72,287	$ 129,927
Accounts receivable, net	1,108,134	1,090,533
Inventories, net	1,018,183	936,047
Prepaid expenses and other current assets	315,643	244,360
Total current assets	2,514,247	2,400,867
Intercompany receivable, net	0	0
Investments in consolidated subsidiaries	0	0
Deferred income taxes, net	0	0
Property and equipment, net	260,376	280,407
Goodwill	2,490,590	2,491,779	$ 1,251,986
Intangibles, net	1,125,540	1,334,366
Other assets, net	2,059	1,243
Total assets	6,392,812	6,508,662
Current liabilities:
Accounts payable	822,931	880,872
Accrued expenses	599,155	611,539
Current portions of long-term debt/obligations	18,689	19,661
Total current liabilities	1,440,775	1,512,072
Intercompany payable, net	0	0
Borrowings under revolving lines of credit, net	80,961	92,442
Long-term debt, net	2,494,623	2,494,725
Deferred income taxes, net	103,913	106,994
Long-term obligations under equipment financing and other, net	4,609	13,639
Other long-term liabilities	6,383	5,290
Total liabilities	4,131,264	4,225,162
Convertible preferred stock	399,195	399,195
Total stockholders' equity	1,862,353	1,884,305	$ 1,781,806	$ 1,323,827
Total liabilities and stockholders' equity	6,392,812	6,508,662
Reportable Legal Entities [Member] | Parent Company [Member]
Current assets:
Cash and cash equivalents	0	0
Accounts receivable, net	0	0
Inventories, net	0	0
Prepaid expenses and other current assets	15,788	23,711
Total current assets	15,788	23,711
Intercompany receivable, net	0	0
Investments in consolidated subsidiaries	6,365,732	6,109,325
Deferred income taxes, net	17,123	22,475
Property and equipment, net	20,507	18,929
Goodwill	0	0
Intangibles, net	0	0
Other assets, net	2,059	1,243
Total assets	6,421,209	6,175,683
Current liabilities:
Accounts payable	51,142	24,154
Accrued expenses	80,556	41,448
Current portions of long-term debt/obligations	9,700	9,700
Total current liabilities	141,398	75,302
Intercompany payable, net	1,523,640	1,322,156
Borrowings under revolving lines of credit, net	0	0
Long-term debt, net	2,494,623	2,494,725
Deferred income taxes, net	0	0
Long-term obligations under equipment financing and other, net	0	0
Other long-term liabilities	0	0
Total liabilities	4,159,661	3,892,183
Convertible preferred stock	399,195	399,195
Total stockholders' equity	1,862,353	1,884,305
Total liabilities and stockholders' equity	6,421,209	6,175,683
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Current assets:
Cash and cash equivalents	65,213	136,499
Accounts receivable, net	1,067,109	1,051,410
Inventories, net	988,925	907,605
Prepaid expenses and other current assets	291,736	214,011
Total current assets	2,412,983	2,309,525
Intercompany receivable, net	1,561,217	1,361,615
Investments in consolidated subsidiaries	0	0
Deferred income taxes, net	0	0
Property and equipment, net	229,267	250,517
Goodwill	2,461,212	2,461,725
Intangibles, net	1,123,768	1,332,104
Other assets, net	0	0
Total assets	7,788,447	7,715,486
Current liabilities:
Accounts payable	745,820	843,907
Accrued expenses	504,150	564,331
Current portions of long-term debt/obligations	8,989	9,961
Total current liabilities	1,258,959	1,418,199
Intercompany payable, net	0	0
Borrowings under revolving lines of credit, net	80,961	89,352
Long-term debt, net	0	0
Deferred income taxes, net	120,643	128,846
Long-term obligations under equipment financing and other, net	4,609	13,639
Other long-term liabilities	6,301	5,207
Total liabilities	1,471,473	1,655,243
Convertible preferred stock	0	0
Total stockholders' equity	6,316,974	6,060,243
Total liabilities and stockholders' equity	7,788,447	7,715,486
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Current assets:
Cash and cash equivalents	7,074	1,959
Accounts receivable, net	42,165	40,262
Inventories, net	29,258	28,442
Prepaid expenses and other current assets	8,119	6,638
Total current assets	86,616	77,301
Intercompany receivable, net	0	0
Investments in consolidated subsidiaries	0	0
Deferred income taxes, net	0	0
Property and equipment, net	10,602	10,961
Goodwill	29,378	30,054
Intangibles, net	1,772	2,262
Other assets, net	0	0
Total assets	128,368	120,578
Current liabilities:
Accounts payable	27,109	22,482
Accrued expenses	14,449	5,760
Current portions of long-term debt/obligations	0	0
Total current liabilities	41,558	28,242
Intercompany payable, net	37,577	39,459
Borrowings under revolving lines of credit, net	0	3,090
Long-term debt, net	0	0
Deferred income taxes, net	393	622
Long-term obligations under equipment financing and other, net	0	0
Other long-term liabilities	82	83
Total liabilities	79,610	71,496
Convertible preferred stock	0	0
Total stockholders' equity	48,758	49,082
Total liabilities and stockholders' equity	128,368	120,578
Eliminations and Other [Member]
Current assets:
Cash and cash equivalents	0	(8,531)
Accounts receivable, net	(1,140)	(1,139)
Inventories, net	0	0
Prepaid expenses and other current assets	0	0
Total current assets	(1,140)	(9,670)
Intercompany receivable, net	(1,561,217)	(1,361,615)
Investments in consolidated subsidiaries	(6,365,732)	(6,109,325)
Deferred income taxes, net	(17,123)	(22,475)
Property and equipment, net	0	0
Goodwill	0	0
Intangibles, net	0	0
Other assets, net	0	0
Total assets	(7,945,212)	(7,503,085)
Current liabilities:
Accounts payable	(1,140)	(9,671)
Accrued expenses	0	0
Current portions of long-term debt/obligations	0	0
Total current liabilities	(1,140)	(9,671)
Intercompany payable, net	(1,561,217)	(1,361,615)
Borrowings under revolving lines of credit, net	0	0
Long-term debt, net	0	0
Deferred income taxes, net	(17,123)	(22,474)
Long-term obligations under equipment financing and other, net	0	0
Other long-term liabilities	0	0
Total liabilities	(1,579,480)	(1,393,760)
Convertible preferred stock	0	0
Total stockholders' equity	(6,365,732)	(6,109,325)
Total liabilities and stockholders' equity	$ (7,945,212)	$ (7,503,085)