Revenue (Impact of ASC 606 Adoption) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 30, 2019	Mar. 31, 2018	Mar. 30, 2019	Mar. 31, 2018	Sep. 30, 2018	Sep. 29, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 233,046	$ 191,323	$ 436,227	$ 385,485
Cost of sales	145,696	116,498	270,572	232,732
Gross profit	87,350	74,825	165,655	152,753
Selling and marketing	33,395	30,597	65,484	62,625
Income tax provision (benefit)	2,916	1,738	3,612	(21,943)
Net income	14,160	$ 8,438	24,661	$ 41,589
Assets
Accounts receivable, net	117,349		117,349		$ 117,762	$ 122,243
Unbilled accounts receivable, net	71,175		71,175		62,472	70,474
Inventories, net	179,071		179,071		155,836	139,109
Prepaid expenses and other current assets	32,307		32,307		29,223	24,572
Other long-term assets	3,900		3,900		3,323	2,263
Deferred income taxes	4,058		4,058		3,892	3,249
Liabilities and Shareholders' Equity
Advance payments from customers	102,033		102,033		93,699	80,131
Accrued income taxes	5,709		5,709			4,928
Other accrued liabilities	34,494		34,494		16,642	19,146
Deferred income taxes	50,659		50,659		45,254	46,482
Other long-term liabilities	12,508		12,508		11,883	4,894
Accumulated other comprehensive income (loss)	(4,618)		(4,618)			1,476
Retained earnings	308,279		308,279		$ 294,358	300,585
Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	224,665		423,103
Cost of sales	140,312		260,732
Gross profit	84,353		162,371
Selling and marketing	33,239		65,491
Income tax provision (benefit)	2,422		3,037
Net income	11,813		21,945
Assets
Accounts receivable, net	121,678		121,678
Unbilled accounts receivable, net	79,819		79,819
Inventories, net	160,914		160,914
Prepaid expenses and other current assets	28,216		28,216
Other long-term assets	2,794		2,794
Deferred income taxes	3,417		3,417
Liabilities and Shareholders' Equity
Advance payments from customers	100,589		100,589
Accrued income taxes	5,861		5,861
Other accrued liabilities	20,855		20,855
Deferred income taxes	51,886		51,886
Other long-term liabilities	11,748		11,748
Accumulated other comprehensive income (loss)	(4,687)		(4,687)
Retained earnings	311,790		311,790
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	8,381		13,124
Cost of sales	5,384		9,840
Gross profit	2,997		3,284
Selling and marketing	156		(7)
Income tax provision (benefit)	494		575
Net income	2,347		2,716
Assets
Accounts receivable, net	(4,329)		(4,329)			(4,481)
Unbilled accounts receivable, net	(8,644)		(8,644)			(8,002)
Inventories, net	18,157		18,157			16,727
Prepaid expenses and other current assets	4,091		4,091			4,651
Other long-term assets	1,106		1,106			1,060
Deferred income taxes	641		641			643
Liabilities and Shareholders' Equity
Advance payments from customers	1,444		1,444			13,568
Accrued income taxes	(152)		(152)
Other accrued liabilities	13,639		13,639			(2,504)
Deferred income taxes	(1,227)		(1,227)			(1,228)
Other long-term liabilities	760		760			6,989
Accumulated other comprehensive income (loss)	69		69
Retained earnings	$ (3,511)		$ (3,511)			$ (6,227)