Segment Reporting (Schedule of Revenues by Geographic Region) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues from External Customers and Long-Lived Assets [Line Items]
Total Agency segment revenues	$ 351,224	$ 94,201	$ 335,817	$ 156,811	$ 339,201	$ 107,864	$ 304,868	$ 101,745	$ 938,053	$ 853,678	$ 768,492
UNITED STATES
Revenues from External Customers and Long-Lived Assets [Line Items]
Total Agency segment revenues									397,113	352,450	338,162
UNITED KINGDOM
Revenues from External Customers and Long-Lived Assets [Line Items]
Total Agency segment revenues									271,505	230,304	221,716
CHINA
Revenues from External Customers and Long-Lived Assets [Line Items]
Total Agency segment revenues									165,066	153,909	108,399
FRANCE
Revenues from External Customers and Long-Lived Assets [Line Items]
Total Agency segment revenues									48,032	46,891	40,972
SWITZERLAND
Revenues from External Customers and Long-Lived Assets [Line Items]
Total Agency segment revenues									46,226	41,150	41,061
Other Countries - Revenue [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total Agency segment revenues									19,116	39,465	25,961
Reconciling Items Member
Revenues from External Customers and Long-Lived Assets [Line Items]
Total Agency segment revenues									$ 9,005	$ 10,491	$ 7,779