Segment, Enterprise-Wide, and Geographical Area Information Segment and Geographical Area Information (External Revenue and Long-Lived Assets) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Revenues from External Customers and Long-Lived Assets [Line Items]
External revenue	$ 173,482	$ 160,952	$ 517,766	$ 487,679
Long-lived assets	99,007		99,007		84,022
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
External revenue	124,998	114,021	372,746	346,442
Long-lived assets	77,713		77,713		60,371
Non United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
External revenue	48,484	46,931	145,020	141,237
Long-lived assets	21,294		21,294		23,651
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
External revenue	14,148	13,711	41,316	42,160
Long-lived assets	6,611		6,611		7,435
Europe excluding the United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
External revenue	14,666	11,643	41,826	35,980
Long-lived assets	2,061		2,061		2,356
Australia
Revenues from External Customers and Long-Lived Assets [Line Items]
External revenue	8,041	10,090	26,569	29,230
Long-lived assets	1,184		1,184		1,402
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
External revenue	7,603	7,605	23,151	22,351
Long-lived assets	1,400		1,400		1,773
Asia excluding Japan
Revenues from External Customers and Long-Lived Assets [Line Items]
External revenue	2,580	2,420	7,836	7,156
Long-lived assets	9,863		9,863		10,445
Japan
Revenues from External Customers and Long-Lived Assets [Line Items]
External revenue	783	924	2,400	2,889
Long-lived assets	45		45		84
Other
Revenues from External Customers and Long-Lived Assets [Line Items]
External revenue	663	538	1,922	1,471
Long-lived assets	$ 130		$ 130		$ 156