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Acquisitions - Additional Information (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
company
Dec. 31, 2013
Dec. 31, 2012
Business Acquisition [Line Items]      
Goodwill $ 251,868,000us-gaap_Goodwill $ 231,838,000us-gaap_Goodwill $ 220,955,000us-gaap_Goodwill
Series of Business Acquisitions      
Business Acquisition [Line Items]      
Number of companies acquired 3us-gaap_NumberOfBusinessesAcquired
/ us-gaap_BusinessAcquisitionAxis
= us-gaap_SeriesOfIndividuallyImmaterialBusinessAcquisitionsMember		    
Aggregate purchase price 40,000,000us-gaap_BusinessCombinationConsiderationTransferred1
/ us-gaap_BusinessAcquisitionAxis
= us-gaap_SeriesOfIndividuallyImmaterialBusinessAcquisitionsMember		    
Goodwill $ 20,000,000us-gaap_Goodwill
/ us-gaap_BusinessAcquisitionAxis
= us-gaap_SeriesOfIndividuallyImmaterialBusinessAcquisitionsMember