Operating Segments and Enterprise-Wide Information - Operating Results by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 26, 2020	Sep. 28, 2019	Sep. 26, 2020	Sep. 28, 2019
Segment Reporting Information [Line Items]
Revenues	$ 177,996	$ 140,604	$ 496,573	$ 410,835
Gross profit	$ 76,749	$ 55,318	$ 210,306	$ 163,191
Gross margin	43.10%	39.30%	42.40%	39.70%
Probe Cards
Segment Reporting Information [Line Items]
Revenues	$ 150,773	$ 116,447	$ 419,272	$ 338,187
Systems
Segment Reporting Information [Line Items]
Revenues	27,223	24,157	77,301	72,648
Corporate and Other
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Gross profit	$ (6,457)	$ (5,824)	$ (19,219)	$ (17,425)
Gross margin	0.00%	0.00%	0.00%	0.00%
Operating Segments | Probe Cards
Segment Reporting Information [Line Items]
Revenues	$ 150,773	$ 116,447	$ 419,272	$ 338,187
Gross profit	$ 69,641	$ 48,127	$ 191,907	$ 141,913
Gross margin	46.20%	41.30%	45.80%	42.00%
Operating Segments | Systems
Segment Reporting Information [Line Items]
Revenues	$ 27,223	$ 24,157	$ 77,301	$ 72,648
Gross profit	$ 13,565	$ 13,015	$ 37,618	$ 38,703
Gross margin	49.80%	53.90%	48.70%	53.30%