Revenue from Contracts with Customers (ASC 606) - Impact of New Revenue Guidance on Financial Statement (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018	Dec. 31, 2015
Balance Sheet
Cash, cash equivalent and marketable securities	$ 18,075								$ 18,075			$ 42,915
Accounts receivable, net	29,663				$ 28,316				29,663	$ 28,316		28,316
Inventories	22,942				22,732				22,942	22,732		22,732
Prepaid expenses and other current assets	3,494				3,816				3,494	3,816		3,816
Long-lived assets	95,979								95,979			92,513
Other non-current assets	6,736				5,927				6,736	5,927		5,927
Contract assets, long-term	6,971				0				6,971	0		5,963
Non-current deferred income tax asset	226				20				226	20		222
Total assets	187,652				196,239				187,652	196,239		205,609
Accounts payable, accrued expenses, and other current liabilities	46,817								46,817			36,430
Deferred revenue, current	0				6,919				0	6,919		0
Contract liabilities	9,193				0				9,193	0		11,039
Long-term contract liabilities	9,070				0				9,070	0		7,998
Other long-term liabilities	3,620								3,620			2,653
Long-term debt, excluding current portion	19,437				44,572				19,437	44,572		44,572
Total liabilities	88,137				90,574				88,137	90,574		102,692
Accumulated deficit	(15,397)				(4,417)				(15,397)	(4,417)		(7,165)
Common stock, additional paid-in capital, and accumulated other comprehensive loss	114,912								114,912			110,082
Total stockholders’ equity	99,515				105,665				99,515	105,665	$ 106,502	102,917	$ 118,176
Total liabilities and stockholders’ equity	187,652				196,239				187,652	196,239		205,609
Consolidated Statement of Operations
Net sales									170,761	160,088	176,122
Research and development									14,951	15,858	16,030
Sales, marketing and support									34,910	33,896	33,942
General and administrative									27,964	28,932	28,172
Total costs and expenses									177,925	168,852	177,444
Loss from operations	(1,877)	$ (920)	$ (1,187)	$ (3,180)	(1,125)	$ (1,801)	$ (1,368)	$ (4,470)	(7,164)	(8,764)	(1,322)
Other income (expense), net									(503)	(1,174)	(648)
Loss before income tax expense									(7,667)	(9,938)	(1,970)
Income tax expense									565	1,096	5,547
Net loss	(1,822)	(1,174)	(1,343)	(3,893)	(1,685)	(2,438)	(2,026)	(4,885)	$ (8,232)	(11,034)	(7,517)
Net loss per common share, basic and diluted (in dollars per share)									$ (0.48)
Statement of Cash Flows
Net loss	(1,822)	$ (1,174)	$ (1,343)	$ (3,893)	(1,685)	$ (2,438)	$ (2,026)	$ (4,885)	$ (8,232)	(11,034)	(7,517)
Non-cash adjustments to reconcile net loss to net cash used in operating activities									15,694
Accounts receivable and inventories									(2,357)
Prepaid expenses, other assets, and contract assets									(2,340)
Deferred revenue, contract liabilities, and long-term contract liabilities									(498)	(138)	1,474
Accounts payable, accrued compensation, warranty, other, and other long-term liabilities									2,917
Net cash provided by operating activities									5,184	11,527	18,699
Current contract assets	3,566				0				3,566	0		3,205
Non-current contract assets	6,971				0				6,971	0		5,963
Contract liabilities	9,193				0				9,193	0		11,039
Long-term contract liabilities	9,070				0				9,070	0		$ 7,998
Pro forma as if the previous accounting guidance had been in effect
Balance Sheet
Cash, cash equivalent and marketable securities	18,075				42,915				18,075	42,915
Accounts receivable, net	29,663				28,316				29,663	28,316
Inventories	22,942				22,732				22,942	22,732
Prepaid expenses and other current assets	3,494				3,816				3,494	3,816
Long-lived assets	95,979				92,513				95,979	92,513
Other non-current assets	6,736				5,927				6,736	5,927
Contract assets, long-term	0				0				0	0
Non-current deferred income tax asset	86				20				86	20
Total assets	176,975				196,239				176,975	196,239
Accounts payable, accrued expenses, and other current liabilities	46,817				36,430				46,817	36,430
Deferred revenue, current	4,833				6,919				4,833	6,919
Contract liabilities	0				0				0	0
Long-term contract liabilities	0				0				0	0
Other long-term liabilities	3,620				2,653				3,620	2,653
Long-term debt, excluding current portion	19,437				44,572				19,437	44,572
Total liabilities	74,707				90,574				74,707	90,574
Accumulated deficit	(12,644)				(4,417)				(12,644)	(4,417)
Common stock, additional paid-in capital, and accumulated other comprehensive loss	114,912				110,082				114,912	110,082
Total stockholders’ equity	102,268				105,665				102,268	105,665
Total liabilities and stockholders’ equity	176,975				196,239				176,975	196,239
Consolidated Statement of Operations
Net sales									172,072
Research and development									14,951
Sales, marketing and support									35,022
General and administrative									27,964
Total costs and expenses									179,294
Loss from operations									(7,222)
Other income (expense), net									(503)
Loss before income tax expense									(7,725)
Income tax expense									502
Net loss									$ (8,227)
Net loss per common share, basic and diluted (in dollars per share)									$ (0.48)
Statement of Cash Flows
Net loss									$ (8,227)
Non-cash adjustments to reconcile net loss to net cash used in operating activities									15,631
Accounts receivable and inventories									(2,357)
Prepaid expenses, other assets, and contract assets									(971)
Deferred revenue, contract liabilities, and long-term contract liabilities									(1,809)
Accounts payable, accrued compensation, warranty, other, and other long-term liabilities									2,917
Net cash provided by operating activities									5,184
Current contract assets	0				0				0	0
Non-current contract assets	0				0				0	0
Contract liabilities	0				0				0	0
Long-term contract liabilities	$ 0				$ 0				0	0
Product
Consolidated Statement of Operations
Net sales									63,277	56,968	73,075
Cost of product and service sales									39,510	37,474	46,334
Product | Pro forma as if the previous accounting guidance had been in effect
Consolidated Statement of Operations
Net sales									64,588
Cost of product and service sales									40,767
Service
Consolidated Statement of Operations
Net sales									107,484	103,120	103,047
Cost of product and service sales									60,590	$ 52,692	$ 52,966
Service | Pro forma as if the previous accounting guidance had been in effect
Consolidated Statement of Operations
Net sales									107,484
Cost of product and service sales									$ 60,590