Schedule of Tangible Long-Lived Assets by Geographic Area (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenues from External Customers and Long-Lived Assets
Long-lived Assets	$ 531,897	$ 494,379	$ 424,098	$ 331,312
United States
Revenues from External Customers and Long-Lived Assets
Long-lived Assets	411,957	408,244	360,961
United Kingdom
Revenues from External Customers and Long-Lived Assets
Long-lived Assets	69,309	60,369	43,816
Other countries
Revenues from External Customers and Long-Lived Assets
Long-lived Assets	$ 50,631	$ 25,766	$ 19,321

Amount, net of accumulated depreciation, depletion and amortization, of long-lived physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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