Enterprise-Wide Disclosures - (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from External Customers	$ 424,415		$ 396,275		$ 331,407
Long-lived Assets	6,916		7,969		8,364
Deferred Tax Assets, Net of Valuation Allowance	8,485		8,368		7,261
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from External Customers	355,022		328,422		269,071
Long-lived Assets	6,102		7,119		7,730
Deferred Tax Assets, Net of Valuation Allowance	8,485		8,368		7,261
Belgium [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from External Customers	41,957	[1]	43,011	[1]	41,317	[1]
Other European Countries [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from External Customers	26,653		23,969		19,396
Europe [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived Assets	814		850		634
Deferred Tax Assets, Net of Valuation Allowance	0		0		0
Other Country [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from External Customers	783		873		1,623
Deferred Tax Assets, Net of Valuation Allowance	$ 0		$ 0		$ 0

[1] Revenue for Belgium has been disclosed separately as it exceeds 10% of consolidated revenue for certain of the years presented