Stock-Based Compensation - Stock Compensation Expense (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
	Nov. 07, 2016	Mar. 31, 2018	Apr. 01, 2017	Mar. 31, 2018	Apr. 01, 2017
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated stock-based compensation expense		$ 7,235	$ 4,868	$ 12,702	$ 13,034
Capitalized share-based compensation costs		1,200	900	2,200	1,700
Amortization of capitalized share-based compensation expenses		1,000	800	2,000	1,500
Total compensation cost, unvested stock-based awards granted but not yet recognized		51,100		$ 51,100
Total compensation cost, weighted-average period of amortization (in years)				1 year 7 months 17 days
Period End
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Capitalized share-based compensation costs				$ 1,400	1,000
Rofin-Sinar
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Cash settlement paid for Rofin employee stock options	$ 15,290
Total estimated merger consideration for canceled of options held by Rofin employees	11,100
Post merger stock compensation expense	$ 4,152
Cost of sales
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated stock-based compensation expense		1,018	778	2,006	1,738
Research and development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated stock-based compensation expense		872	597	1,540	1,650
Selling, general and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated stock-based compensation expense		6,520	5,308	11,940	12,950
Income tax benefit
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Income tax benefit		$ (1,175)	$ (1,815)	$ (2,784)	$ (3,304)