Supplemental Guarantor Information - Condensed Consolidating Balance Sheets (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017
Current assets:
Cash and cash equivalents	$ 645		$ 129,927	$ 16,000
Accounts receivable, net	869,760		1,090,533	832,823
Inventories, net	1,031,183		936,047	1,005,577
Prepaid expenses and other current assets	332,100		244,360	240,315
Total current assets	2,233,688		2,400,867	2,094,715
Intercompany receivable, net	0		0	0
Investments in consolidated subsidiaries	0		0	0
Deferred income taxes, net	0		0	0
Property and equipment, net	271,022		280,407	294,222
Goodwill	2,490,326		2,491,779	2,381,620		$ 1,251,986
Intangibles, net	1,229,949		1,334,366	1,410,302
Other assets, net	1,243		1,243	1,511
Total assets	6,226,228		6,508,662	6,182,370
Current liabilities:
Accounts payable	510,434		880,872	593,559
Accrued expenses	453,889		611,539	348,050
Current portions of long-term debt/obligations	19,988		19,661	19,597
Total current liabilities	984,311		1,512,072	961,206
Intercompany payable, net	0		0	0
Borrowings under revolving lines of credit, net	416,614		92,442	424,528
Long-term debt, net	2,494,673		2,494,725	2,493,889
Deferred income taxes, net	110,064		106,994	91,101
Long-term obligations under equipment financing and other, net	8,527		13,639	18,313
Other long-term liabilities	5,702		5,290	10,617
Total liabilities	4,019,891		4,225,162	3,999,654
Convertible preferred stock	399,195		399,195	399,195
Total stockholders' equity	1,807,142	$ 1,874,176	1,884,305	1,783,521	$ 1,852,344	$ 1,781,806
Total liabilities and stockholders' equity	6,226,228		6,508,662	6,182,370
Reportable Legal Entities [Member] | Parent Company [Member]
Current assets:
Cash and cash equivalents	0		0	0
Accounts receivable, net	0		0	0
Inventories, net	0		0	0
Prepaid expenses and other current assets	37,086		23,711	35,646
Total current assets	37,086		23,711	35,646
Intercompany receivable, net	0		0	0
Investments in consolidated subsidiaries	6,240,394		6,109,325	5,949,437
Deferred income taxes, net	21,170		22,475	19,902
Property and equipment, net	20,210		18,929	10,761
Goodwill	0		0	0
Intangibles, net	0		0	0
Other assets, net	1,243		1,243	1,243
Total assets	6,320,103		6,175,683	6,016,989
Current liabilities:
Accounts payable	7,758		24,154	21,042
Accrued expenses	28,685		41,448	61,788
Current portions of long-term debt/obligations	9,700		9,700	9,700
Total current liabilities	46,143		75,302	92,530
Intercompany payable, net	1,572,950		1,322,156	1,247,040
Borrowings under revolving lines of credit, net	0		0	0
Long-term debt, net	2,494,673		2,494,725	2,493,889
Deferred income taxes, net	0		0	0
Long-term obligations under equipment financing and other, net	0		0	0
Other long-term liabilities	0		0	814
Total liabilities	4,113,766		3,892,183	3,834,273
Convertible preferred stock	399,195		399,195	399,195
Total stockholders' equity	1,807,142		1,884,305	1,783,521
Total liabilities and stockholders' equity	6,320,103		6,175,683	6,016,989
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Current assets:
Cash and cash equivalents	312		136,499	18,746
Accounts receivable, net	850,149		1,051,410	816,058
Inventories, net	1,001,561		907,605	978,852
Prepaid expenses and other current assets	287,869		214,011	199,947
Total current assets	2,139,891		2,309,525	2,013,603
Intercompany receivable, net	1,610,041		1,361,615	1,284,455
Investments in consolidated subsidiaries	0		0	0
Deferred income taxes, net	0		0	0
Property and equipment, net	240,948		250,517	272,866
Goodwill	2,461,212		2,461,725	2,351,447
Intangibles, net	1,227,975		1,332,104	1,407,772
Other assets, net	0		0	268
Total assets	7,680,067		7,715,486	7,330,411
Current liabilities:
Accounts payable	494,376		843,907	572,222
Accrued expenses	418,963		564,331	281,265
Current portions of long-term debt/obligations	10,288		9,961	9,897
Total current liabilities	923,627		1,418,199	863,384
Intercompany payable, net	0		0	0
Borrowings under revolving lines of credit, net	414,369		89,352	424,528
Long-term debt, net	0		0	0
Deferred income taxes, net	130,932		128,846	110,614
Long-term obligations under equipment financing and other, net	8,527		13,639	18,313
Other long-term liabilities	5,621		5,207	9,729
Total liabilities	1,483,076		1,655,243	1,426,568
Convertible preferred stock	0		0	0
Total stockholders' equity	6,196,991		6,060,243	5,903,843
Total liabilities and stockholders' equity	7,680,067		7,715,486	7,330,411
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Current assets:
Cash and cash equivalents	333		1,959	8,196
Accounts receivable, net	20,751		40,262	17,905
Inventories, net	29,622		28,442	26,725
Prepaid expenses and other current assets	7,145		6,638	4,722
Total current assets	57,851		77,301	57,548
Intercompany receivable, net	0		0	0
Investments in consolidated subsidiaries	0		0	0
Deferred income taxes, net	0		0	0
Property and equipment, net	9,864		10,961	10,595
Goodwill	29,114		30,054	30,173
Intangibles, net	1,974		2,262	2,530
Other assets, net	0		0	0
Total assets	98,803		120,578	100,846
Current liabilities:
Accounts payable	9,440		22,482	12,377
Accrued expenses	6,241		5,760	4,997
Current portions of long-term debt/obligations	0		0	0
Total current liabilities	15,681		28,242	17,374
Intercompany payable, net	37,091		39,459	37,415
Borrowings under revolving lines of credit, net	2,245		3,090	0
Long-term debt, net	0		0	0
Deferred income taxes, net	302		622	389
Long-term obligations under equipment financing and other, net	0		0	0
Other long-term liabilities	81		83	74
Total liabilities	55,400		71,496	55,252
Convertible preferred stock	0		0	0
Total stockholders' equity	43,403		49,082	45,594
Total liabilities and stockholders' equity	98,803		120,578	100,846
Eliminations and Other [Member]
Current assets:
Cash and cash equivalents	0		(8,531)	(10,942)
Accounts receivable, net	(1,140)		(1,139)	(1,140)
Inventories, net	0		0	0
Prepaid expenses and other current assets	0		0	0
Total current assets	(1,140)		(9,670)	(12,082)
Intercompany receivable, net	(1,610,041)		(1,361,615)	(1,284,455)
Investments in consolidated subsidiaries	(6,240,394)		(6,109,325)	(5,949,437)
Deferred income taxes, net	(21,170)		(22,475)	(19,902)
Property and equipment, net	0		0	0
Goodwill	0		0	0
Intangibles, net	0		0	0
Other assets, net	0		0	0
Total assets	(7,872,745)		(7,503,085)	(7,265,876)
Current liabilities:
Accounts payable	(1,140)		(9,671)	(12,082)
Accrued expenses	0		0	0
Current portions of long-term debt/obligations	0		0	0
Total current liabilities	(1,140)		(9,671)	(12,082)
Intercompany payable, net	(1,610,041)		(1,361,615)	(1,284,455)
Borrowings under revolving lines of credit, net	0		0	0
Long-term debt, net	0		0	0
Deferred income taxes, net	(21,170)		(22,474)	(19,902)
Long-term obligations under equipment financing and other, net	0		0	0
Other long-term liabilities	0		0	0
Total liabilities	(1,632,351)		(1,393,760)	(1,316,439)
Convertible preferred stock	0		0	0
Total stockholders' equity	(6,240,394)		(6,109,325)	(5,949,437)
Total liabilities and stockholders' equity	$ (7,872,745)		$ (7,503,085)	$ (7,265,876)