Revenue (Disaggregation) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 30, 2019	Mar. 31, 2018	Mar. 30, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 233,046	$ 191,323	$ 436,227	$ 385,485
Americas
Disaggregation of Revenue [Line Items]
Revenue	90,013		166,902
Europe
Disaggregation of Revenue [Line Items]
Revenue	57,189		111,164
Asia
Disaggregation of Revenue [Line Items]
Revenue	85,844		158,161
Point-in-time
Disaggregation of Revenue [Line Items]
Revenue	180,374		337,614
Over time
Disaggregation of Revenue [Line Items]
Revenue	52,672		98,613
Product
Disaggregation of Revenue [Line Items]
Revenue	206,690	165,453	381,769	334,694
Service
Disaggregation of Revenue [Line Items]
Revenue	26,356	25,870	54,458	50,791
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue	(361)	(457)	(690)	(457)
Intersegment Eliminations | Americas
Disaggregation of Revenue [Line Items]
Revenue	(361)		(690)
Intersegment Eliminations | Europe
Disaggregation of Revenue [Line Items]
Revenue	0		0
Intersegment Eliminations | Asia
Disaggregation of Revenue [Line Items]
Revenue	0		0
Intersegment Eliminations | Point-in-time
Disaggregation of Revenue [Line Items]
Revenue	(361)		(690)
Intersegment Eliminations | Over time
Disaggregation of Revenue [Line Items]
Revenue	0		0
Intersegment Eliminations | Product
Disaggregation of Revenue [Line Items]
Revenue	(361)		(690)
Intersegment Eliminations | Service
Disaggregation of Revenue [Line Items]
Revenue	0		0
Operating Segments | Test & Simulation
Disaggregation of Revenue [Line Items]
Revenue	151,032	110,238	276,592	228,441
Operating Segments | Test & Simulation | Americas
Disaggregation of Revenue [Line Items]
Revenue	51,399		90,888
Operating Segments | Test & Simulation | Europe
Disaggregation of Revenue [Line Items]
Revenue	29,536		58,163
Operating Segments | Test & Simulation | Asia
Disaggregation of Revenue [Line Items]
Revenue	70,097		127,541
Operating Segments | Test & Simulation | Point-in-time
Disaggregation of Revenue [Line Items]
Revenue	98,360		177,979
Operating Segments | Test & Simulation | Over time
Disaggregation of Revenue [Line Items]
Revenue	52,672		98,613
Operating Segments | Test & Simulation | Product
Disaggregation of Revenue [Line Items]
Revenue	126,511		225,419
Operating Segments | Test & Simulation | Service
Disaggregation of Revenue [Line Items]
Revenue	24,521		51,173
Operating Segments | Sensors
Disaggregation of Revenue [Line Items]
Revenue	82,375	$ 81,542	160,325	$ 157,501
Operating Segments | Sensors | Americas
Disaggregation of Revenue [Line Items]
Revenue	38,975		76,704
Operating Segments | Sensors | Europe
Disaggregation of Revenue [Line Items]
Revenue	27,653		53,001
Operating Segments | Sensors | Asia
Disaggregation of Revenue [Line Items]
Revenue	15,747		30,620
Operating Segments | Sensors | Point-in-time
Disaggregation of Revenue [Line Items]
Revenue	82,375		160,325
Operating Segments | Sensors | Over time
Disaggregation of Revenue [Line Items]
Revenue	0		0
Operating Segments | Sensors | Product
Disaggregation of Revenue [Line Items]
Revenue	80,540		157,040
Operating Segments | Sensors | Service
Disaggregation of Revenue [Line Items]
Revenue	$ 1,835		$ 3,285