Schedule of Tangible Long-Lived Assets by Geographic Area (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues from External Customers and Long-Lived Assets
Long-lived Assets	$ 629,987	$ 546,651	$ 494,379
United States
Revenues from External Customers and Long-Lived Assets
Long-lived Assets	469,450	418,869
United Kingdom
Revenues from External Customers and Long-Lived Assets
Long-lived Assets	92,813	77,827
Other countries
Revenues from External Customers and Long-Lived Assets
Long-lived Assets	$ 67,724	$ 49,955