Enterprise-Wide Disclosures - Schedule of Revenue from External Customers and Long-Lived Assets, by Geographical Areas (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 29, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jul. 01, 2016	Apr. 01, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Enterprise-Wide Disclosures
Revenue		$ 74,644	$ 74,039	$ 75,521	$ 77,006	$ 77,492	$ 78,065	$ 83,486	$ 85,850	$ 301,210	$ 324,893	$ 369,478
Long-lived assets		6,996				5,863				6,996	5,863	5,488
Deferred tax assets, net of valuation allowance		3,861				6,886				3,861	6,886	6,406
United States [Member]
Enterprise-Wide Disclosures
Revenue										219,886	253,955	301,826
Long-lived assets		5,206				4,280				5,206	4,280	4,208
Deferred tax assets, net of valuation allowance		3,810				6,886				3,810	6,886	6,352
Europe [Member]
Enterprise-Wide Disclosures
Deferred tax assets, net of valuation allowance		51								51		54
Belgium [Member]
Enterprise-Wide Disclosures
Revenue	[1]									$ 39,347	$ 35,995	$ 35,931
Threshold for Separate Geographic Disclosure										10.00%	10.00%	10.00%
Luxembourg [Member]
Enterprise-Wide Disclosures
Revenue	[1]									$ 36,954	$ 31,441	$ 28,562
Threshold for Separate Geographic Disclosure										10.00%	10.00%	10.00%
Other European Countries [Member]
Enterprise-Wide Disclosures
Revenue										$ 4,824	$ 3,193	$ 2,814
Long-lived assets		782				791				782	791	819
Other Country [Member]
Enterprise-Wide Disclosures
Revenue										199	309	345
Long-lived assets		3								3
Foreign Countries [Member]
Enterprise-Wide Disclosures
Revenue										81,324	70,938	67,652
United Kingdom [Member]
Enterprise-Wide Disclosures
Long-lived assets	[2]	$ 1,005				$ 792				$ 1,005	$ 792	$ 461
Threshold for Long-lived Asset Geographic Disclosure										10.00%	10.00%	10.00%

[1] Revenue for our Belgium and Luxembourg operations has been disclosed separately as it exceeds 10% of consolidated revenue in at least one of the years presented.

[2] Long-lived assets for our United Kingdom operations has been disclosed separately as it exceeds 10% of consolidated long-lived assets in at least one of the years presented.