Supplemental Guarantor Information - Condensed Consolidating Statements of Cash Flows (Detail) - USD ($) $ in Thousands		12 Months Ended
	Jan. 02, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Net cash provided by (used in) operating activities		$ 212,653	$ 539,381	$ 315,200
Investing Activities
Purchases of property and equipment		(57,031)	(46,010)	(39,828)
Acquisition of businesses, net		(163,973)	(2,740,480)	(129,390)
Proceeds from the sale of assets		9,269	2,149	2,233
Intercompany activity		0	0	0
Net cash provided by (used in) investing activities		(211,735)	(2,784,341)	(166,985)
Financing Activities
Borrowings under revolving lines of credit		2,100,085	2,807,741	2,464,128
Repayments under revolving lines of credit		(2,113,982)	(2,707,741)	(2,833,230)
Borrowings under term loan		0	970,000	0
Repayments under term loan		(9,700)	(445,850)	(4,500)
Borrowings under senior notes		0	1,300,000	0
Payment of debt issuance costs	$ (65,300)	(817)	(65,788)	(1,669)
Repayments under equipment financing facilities and other		(10,001)	(11,593)	(10,034)
Proceeds from secondary offering of common stock		0	0	345,503
Proceeds from issuance of convertible preferred stock		0	400,000	0
Payment of stock issuance costs		0	(1,279)	(14,684)
Payment of dividends on preferred stock		(24,000)	(12,978)	0
Proceeds from issuance of common stock related to equity awards		3,314	7,514	11,341
Taxes paid related to net share settlement of equity awards		(3,668)	(3,975)	(392)
Excess tax benefit from stock-based compensation		0	0	2,937
Intercompany activity		0	0	0
Net cash provided by (used in) financing activities		(58,769)	2,236,051	(40,600)
Effect of exchange rate changes on cash and cash equivalents		211	586	(751)
Net increase (decrease) in cash and cash equivalents		(57,640)	(8,323)	106,864
Cash and cash equivalents, beginning of period		129,927	138,250	31,386
Cash and cash equivalents, end of period		72,287	129,927	138,250
Reportable Legal Entities [Member] | Parent Company [Member]
Net cash provided by (used in) operating activities		6,250	(100,302)	(2,466)
Investing Activities
Purchases of property and equipment		(7,156)	(14,354)	(3,517)
Acquisition of businesses, net		0	(2,740,480)	(129,390)
Proceeds from the sale of assets		0	0	0
Intercompany activity		35,777	699,970	(203,163)
Net cash provided by (used in) investing activities		28,621	(2,054,864)	(336,070)
Financing Activities
Borrowings under term loan			970,000
Repayments under term loan		(9,700)	(445,850)	(4,500)
Borrowings under senior notes			1,300,000
Payment of debt issuance costs		(817)	(58,266)	(1,669)
Repayments under equipment financing facilities and other		0	0	0
Proceeds from secondary offering of common stock				345,503
Proceeds from issuance of convertible preferred stock			400,000
Payment of stock issuance costs			(1,279)	(14,684)
Payment of dividends on preferred stock		(24,000)	(12,978)
Proceeds from issuance of common stock related to equity awards		3,314	7,514	11,341
Taxes paid related to net share settlement of equity awards		(3,668)	(3,975)	(392)
Excess tax benefit from stock-based compensation				2,937
Intercompany activity		0	0	0
Net cash provided by (used in) financing activities		(34,871)	2,155,166	338,536
Effect of exchange rate changes on cash and cash equivalents		0	0	0
Net increase (decrease) in cash and cash equivalents		0	0	0
Cash and cash equivalents, beginning of period		0	0	0
Cash and cash equivalents, end of period		0	0	0
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Net cash provided by (used in) operating activities		188,268	638,451	313,396
Investing Activities
Purchases of property and equipment		(48,311)	(29,132)	(34,584)
Acquisition of businesses, net		(163,973)	0	0
Proceeds from the sale of assets		9,256	2,121	2,150
Intercompany activity		0	0	0
Net cash provided by (used in) investing activities		(203,028)	(27,011)	(32,434)
Financing Activities
Borrowings under term loan			0
Repayments under term loan		0	0	0
Borrowings under senior notes			0
Payment of debt issuance costs		0	(7,522)	0
Repayments under equipment financing facilities and other		(10,001)	(11,593)	(10,049)
Proceeds from secondary offering of common stock				0
Proceeds from issuance of convertible preferred stock			0
Payment of stock issuance costs			0	0
Payment of dividends on preferred stock		0	0
Proceeds from issuance of common stock related to equity awards		0	0	0
Taxes paid related to net share settlement of equity awards		0	0	0
Excess tax benefit from stock-based compensation				0
Intercompany activity		(35,629)	(705,625)	209,018
Net cash provided by (used in) financing activities		(56,526)	(624,740)	(168,610)
Effect of exchange rate changes on cash and cash equivalents		0	0	0
Net increase (decrease) in cash and cash equivalents		(71,286)	(13,300)	112,352
Cash and cash equivalents, beginning of period		136,499	149,799	37,447
Cash and cash equivalents, end of period		65,213	136,499	149,799
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Net cash provided by (used in) operating activities		11,339	(680)	4,888
Investing Activities
Purchases of property and equipment		(1,564)	(2,524)	(1,727)
Acquisition of businesses, net		0	0	0
Proceeds from the sale of assets		13	28	83
Intercompany activity		0	0	0
Net cash provided by (used in) investing activities		(1,551)	(2,496)	(1,644)
Financing Activities
Borrowings under term loan			0
Repayments under term loan		0	0	0
Borrowings under senior notes			0
Payment of debt issuance costs		0	0	0
Repayments under equipment financing facilities and other		0	0	15
Proceeds from secondary offering of common stock				0
Proceeds from issuance of convertible preferred stock			0
Payment of stock issuance costs			0	0
Payment of dividends on preferred stock		0	0
Proceeds from issuance of common stock related to equity awards		0	0	0
Taxes paid related to net share settlement of equity awards		0	0	0
Excess tax benefit from stock-based compensation				0
Intercompany activity		(1,826)	2,967	(2,279)
Net cash provided by (used in) financing activities		(4,827)	2,967	(3,787)
Effect of exchange rate changes on cash and cash equivalents		154	586	(751)
Net increase (decrease) in cash and cash equivalents		5,115	377	(1,294)
Cash and cash equivalents, beginning of period		1,959	1,582	2,876
Cash and cash equivalents, end of period		7,074	1,959	1,582
Eliminations and Other [Member]
Net cash provided by (used in) operating activities		6,796	1,912	(618)
Investing Activities
Purchases of property and equipment		0	0	0
Acquisition of businesses, net		0	0	0
Proceeds from the sale of assets		0	0	0
Intercompany activity		(35,777)	(699,970)	203,163
Net cash provided by (used in) investing activities		(35,777)	(699,970)	203,163
Financing Activities
Borrowings under term loan			0
Repayments under term loan		0	0	0
Borrowings under senior notes			0
Payment of debt issuance costs		0	0	0
Repayments under equipment financing facilities and other		0	0	0
Proceeds from secondary offering of common stock				0
Proceeds from issuance of convertible preferred stock			0
Payment of stock issuance costs			0	0
Payment of dividends on preferred stock		0	0
Proceeds from issuance of common stock related to equity awards		0	0	0
Taxes paid related to net share settlement of equity awards		0	0	0
Excess tax benefit from stock-based compensation				0
Intercompany activity		37,455	702,658	(206,739)
Net cash provided by (used in) financing activities		37,455	702,658	(206,739)
Effect of exchange rate changes on cash and cash equivalents		57	0	0
Net increase (decrease) in cash and cash equivalents		8,531	4,600	(4,194)
Cash and cash equivalents, beginning of period		(8,531)	(13,131)	(8,937)
Cash and cash equivalents, end of period		0	(8,531)	(13,131)
Revolving Lines of Credit [Member]
Financing Activities
Borrowings under revolving lines of credit		2,100,085	2,807,741	2,464,128
Repayments under revolving lines of credit		(2,113,982)	(2,707,741)	(2,833,230)
Revolving Lines of Credit [Member] | Reportable Legal Entities [Member] | Parent Company [Member]
Financing Activities
Borrowings under revolving lines of credit		0	0	0
Repayments under revolving lines of credit		0	0	0
Revolving Lines of Credit [Member] | Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Financing Activities
Borrowings under revolving lines of credit		2,049,500	2,766,447	2,445,084
Repayments under revolving lines of credit		(2,060,396)	(2,666,447)	(2,812,663)
Revolving Lines of Credit [Member] | Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Financing Activities
Borrowings under revolving lines of credit		50,585	41,294	19,044
Repayments under revolving lines of credit		(53,586)	(41,294)	(20,567)
Revolving Lines of Credit [Member] | Eliminations and Other [Member]
Financing Activities
Borrowings under revolving lines of credit		0	0	0
Repayments under revolving lines of credit		$ 0	$ 0	$ 0