Fair Value Measurement (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair value disclosures
Contingent consideration, long-term	$ 27,940	$ 17,710
Fair value measured on recurring basis | Total carrying value
Fair value disclosures
Cash and cash equivalents	205,586
Short term investments	69,608
Contingent consideration, long-term	27,940
Fair value measured on recurring basis | Fair Value, Level 1
Fair value disclosures
Cash and cash equivalents	205,586
Short term investments	69,608
Fair value measured on recurring basis | Fair Value, Level 3
Fair value disclosures
Contingent consideration, long-term	$ 27,940