Segment and Geographical Area Information (External Revenue and Long-Lived Assets) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 180,500	$ 173,482	$ 175,428	$ 168,856	$ 170,609	$ 160,952	$ 165,968	$ 160,759	$ 698,266	$ 658,288	$ 631,400
Long-lived assets	104,986				84,022				104,986	84,022
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									500,730	466,947	446,470
Long-lived assets	84,321				60,371				84,321	60,371
United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									56,298	56,794	53,427
Long-lived assets	6,873				7,435				6,873	7,435
Europe excluding the United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									57,580	49,844	49,507
Long-lived assets	1,873				2,356				1,873	2,356
Australia [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									35,289	38,229	39,761
Long-lived assets	1,051				1,402				1,051	1,402
Canada [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									31,845	30,664	27,808
Long-lived assets	1,275				1,773				1,275	1,773
Asia, Excluding Japan [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									13,860	13,765	13,188
Long-lived assets	9,479				10,529				9,479	10,529
Other [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									2,664	2,045	1,239
Long-lived assets	$ 114				$ 156				$ 114	$ 156