Restatement and Revision of Prior Period Quarterly Financial Statements - Restated and Revised Consolidated Balance Sheet Amounts (Unaudited) (Details) - USD ($)	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Accounts receivable	$ 205,386,000	$ 200,799,000	$ 206,242,000	$ 184,713,000	$ 182,464,000	$ 185,659,000	$ 188,247,000	$ 196,251,000
Unbilled revenue	50,016,000	48,645,000	34,264,000	33,771,000	31,516,000	37,982,000	29,202,000	32,457,000
Inventories	40,694,000	54,174,000	34,558,000	36,674,000	36,448,000	52,830,000	43,528,000	42,350,000
Prepaid expenses	18,565,000	22,018,000	23,588,000	23,912,000	18,459,000	11,551,000	11,712,000	12,524,000
Other current assets	37,865,000	31,529,000	20,475,000	20,302,000	24,597,000	31,302,000	17,738,000	16,699,000
Total current assets	383,088,000	384,847,000	342,664,000	329,292,000	324,408,000	340,111,000	322,033,000	321,210,000
Other non-current assets	1,636,000	1,592,000	1,547,000	1,633,000	1,654,000	1,426,000	1,415,000	1,469,000
Total intangibles and other assets	229,836,000	238,473,000	237,926,000	236,794,000	236,923,000	241,003,000	242,025,000	244,903,000
Total assets	649,638,000	660,532,000	616,620,000	600,086,000	593,987,000	613,393,000	597,148,000	599,321,000
Accounts payable	141,164,000	130,406,000	126,382,000	114,616,000	123,012,000	129,869,000	139,914,000	129,894,000
Accrued expenses	34,391,000	32,889,000	27,409,000	32,422,000	29,755,000	20,788,000	16,096,000	23,939,000
Other current liabilities	24,078,000	30,416,000	25,210,000	24,359,000	22,576,000	25,760,000	24,200,000	24,401,000
Deferred revenue	17,620,000	15,709,000	15,392,000	16,885,000	16,402,000	13,409,000	15,180,000	14,589,000
Total current liabilities	217,253,000	209,420,000	212,235,000	206,540,000	211,028,000	210,829,000	207,779,000	207,558,000
Deferred income taxes	12,043,000	9,839,000	9,876,000	9,818,000	10,766,000	10,622,000	11,060,905	9,652,000
Other long-term liabilities	7,399,000	2,595,000	2,625,000	2,697,000	2,564,000	2,591,000	2,995,000	3,129,000
Total liabilities	365,093,000	371,038,000	343,394,000	332,746,000	331,826,000	357,776,000	348,100,000	348,142,000
Accumulated other comprehensive loss	(19,229,000)	(12,922,000)	(15,090,000)	(18,970,000)	(20,949,000)	(15,668,000)	(15,467,000)	(14,077,000)
Retained earnings	124,442,000	124,369,000	117,504,000	113,147,000	108,082,000	103,918,000	99,693,000	101,734,000
Total stockholders' equity	284,545,000	289,494,000	273,226,000	267,340,000	262,161,000	255,617,000	249,048,000	251,179,000	$ 252,432,000	$ 290,260,000
Total liabilities and stockholders' equity	649,638,000	660,532,000	616,620,000	600,086,000	593,987,000	613,393,000	597,148,000	599,321,000
As Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Accounts receivable	206,712,000	203,306,000	208,382,000	187,028,000	182,874,000	190,438,000	193,297,000	200,629,000
Unbilled revenue	49,389,000	50,130,000	36,515,000	35,712,000	32,723,000	40,505,000	27,857,000	34,548,000
Inventories	34,807,000	48,744,000	28,192,000	30,338,000	31,638,000	45,377,000	35,901,000	34,715,000
Prepaid expenses	19,638,000				18,772,000	11,984,000	12,145,000	12,957,000
Other current assets	32,694,000	31,850,000	20,949,000	20,598,000	24,769,000	30,528,000	17,647,000	16,708,000
Total current assets	373,802,000	383,730,000	341,163,000	327,508,000	321,700,000	339,619,000	318,453,000	320,485,000
Other non-current assets	1,382,000	1,312,000	1,267,000	1,353,000	1,374,000
Total intangibles and other assets	229,503,000	238,193,000	237,646,000	236,514,000	236,642,000
Total assets	640,019,000	659,135,000	614,839,000	598,022,000	590,999,000	612,901,000	593,567,000	598,596,000
Accounts payable	134,609,000	129,600,000	125,593,000	114,399,000	121,289,000	129,980,000	136,260,000	130,048,000
Accrued expenses	33,694,000	32,994,000	27,567,000	32,547,000	30,068,000	21,037,000	17,818,000	24,257,000
Other current liabilities	20,956,000	31,589,000	26,163,000	25,231,000	22,971,000
Deferred revenue	13,685,000	11,707,000	11,585,000	12,405,000	12,078,000	9,755,000	11,793,000	10,975,000
Total current liabilities	202,943,000	205,890,000	208,750,000	202,840,000	205,688,000	207,535,000	202,460,000	204,416,000
Deferred income taxes	12,348,000	9,834,000	9,852,000	9,848,000	11,291,000	11,594,000	11,620,000	10,476,000
Other long-term liabilities	6,771,000	1,958,000	1,988,000	2,060,000	1,926,000	2,235,000	2,639,000	2,772,000
Total liabilities	350,461,000	366,866,000	339,248,000	328,439,000	326,373,000	355,098,000	342,984,000	345,466,000
Accumulated other comprehensive loss	(19,079,000)	(12,772,000)	(14,940,000)	(18,820,000)	(20,799,000)	(15,518,000)	(15,853,000)	(14,463,000)
Retained earnings	129,305,000	126,994,000	119,719,000	115,240,000	110,397,000	105,954,000	101,614,000	104,071,000
Total stockholders' equity	289,559,000	292,269,000	275,591,000	269,583,000	264,626,000	257,803,000	250,583,000	253,130,000
Total liabilities and stockholders' equity	640,019,000	659,135,000	614,839,000	598,022,000	590,999,000	612,901,000	593,567,000	598,596,000
Adjustments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Accounts receivable	(1,326,000)	(2,507,000)	(2,140,000)	(2,315,000)	(410,000)	(4,779,000)	(5,050,000)	(4,378,000)
Unbilled revenue	627,000	(1,485,000)	(2,251,000)	(1,941,000)	(1,207,000)	(2,523,000)	1,345,000	(2,091,000)
Inventories	5,887,000	5,430,000	6,366,000	6,336,000	4,810,000	7,453,000	7,627,000	7,635,000
Prepaid expenses	(1,073,000)				(313,000)	(433,000)	(433,000)	(433,000)
Other current assets	5,171,000	(321,000)	(474,000)	(296,000)	(172,000)	774,000	92,000	(8,000)
Total current assets	9,286,000	1,117,000	1,501,000	1,784,000	2,708,000	492,000	3,580,000	725,000
Other non-current assets	254,000	280,000	280,000	280,000	280,000
Total intangibles and other assets	333,000	280,000	280,000	280,000	280,000
Total assets	9,619,000	1,397,000	1,781,000	2,064,000	2,988,000	492,000	3,582,000	725,000
Accounts payable	6,555,000	806,000	789,000	217,000	1,723,000	(111,000)	3,654,000	(154,000)
Accrued expenses	697,000	(105,000)	(158,000)	(125,000)	(313,000)	(249,000)	(1,722,000)	(318,000)
Other current liabilities	3,122,000	(1,173,000)	(953,000)	(872,000)	(395,000)
Deferred revenue	3,935,000	4,002,000	3,807,000	4,480,000	4,324,000	3,654,000	3,387,000	3,614,000
Total current liabilities	14,310,000	3,530,000	3,485,000	3,700,000	5,340,000	3,294,000	5,319,000	3,142,000
Deferred income taxes	(305,000)	5,000	24,000	(30,000)	(525,000)	(972,000)	(559,000)	(824,000)
Other long-term liabilities	628,000	637,000	637,000	637,000	638,000	356,000	356,000	358,000
Total liabilities	14,632,000	4,172,000	4,146,000	4,307,000	5,453,000	2,678,000	5,116,000	2,676,000
Accumulated other comprehensive loss	(150,000)	(150,000)	(150,000)	(150,000)	(150,000)	(150,000)	386,000	386,000
Retained earnings	(4,863,000)	(2,625,000)	(2,215,000)	(2,093,000)	(2,315,000)	(2,036,000)	(1,921,000)	(2,337,000)
Total stockholders' equity	(5,014,000)	(2,775,000)	(2,365,000)	(2,243,000)	(2,465,000)	(2,186,000)	(1,535,000)	(1,951,000)
Total liabilities and stockholders' equity	$ 9,619,000	$ 1,397,000	$ 1,781,000	$ 2,064,000	$ 2,988,000	$ 492,000	$ 3,581,000	$ 725,000