Income Taxes - Deferred Tax Reconciliation (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Assets, Net of Valuation Allowance [Abstract]
Accrued warranty	$ 256	$ 657
Inventory	6,114	3,631
Net operating loss carryforwards	39,534	57,294
Tax credit carryforwards	20,857	22,075
Stock-based compensation	978	1,995
Fixed assets	2,358	1,808
Goodwill and other identified intangibles	1,073	1,603
Deferred revenue	1,201	2,906
Subsidiary service accruals	4,870	1,702
Other	1,308	2,082
Total deferred tax assets	78,549	95,753
Less: valuation allowance	(77,437)	(88,566)
Net deferred tax assets	1,112	7,187
Deferred tax liabilities:
Intangible assets	0	(5,603)
Other	(325)	(467)
Total deferred tax liabilities	(325)	(6,070)
Total net deferred tax assets	$ 787	$ 1,117