Segment Information - Comparative Results of Segment Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Net sales
Net sales	$ 49,675,000	$ 50,725,000	$ 147,078,000	$ 151,140,000
Income from operations
Income from operations	3,682,000	4,182,000	12,858,000	14,926,000
Unallocated Corporate [Member]
Income from operations
Income from operations	(1,553,000)	(1,870,000)	(5,396,000)	(5,099,000)
SLPE [Member]
Net sales
Net sales	17,243,000	17,821,000	51,687,000	54,504,000
SLPE [Member] | Operating Segments [Member]
Income from operations
Income from operations	2,416,000	1,971,000	6,140,000	5,203,000
High Power Group [Member]
Net sales
Net sales	15,224,000	21,699,000	53,366,000	64,461,000
High Power Group [Member] | Operating Segments [Member]
Income from operations
Income from operations	1,028,000	2,838,000	5,914,000	9,676,000
SL-MTI [Member]
Net sales
Net sales	17,208,000	11,205,000	42,025,000	32,175,000
SL-MTI [Member] | Operating Segments [Member]
Income from operations
Income from operations	$ 1,791,000	$ 1,243,000	$ 6,200,000	$ 5,146,000