Acquisitions - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Business Acquisition [Line Items]
Initial consideration	$ 8,300,000	$ 51,000,000	$ 41,600,000
Number of acquisitions completed	1	6	3
Purchase price allocation, identifiable intangible assets	1,600,000	17,200,000	5,800,000
Goodwill	1,211,689,000	1,218,733,000	1,260,035,000
Contingent consideration			4,500,000
Consideration transferred in cash and stock			37,200,000
2014 Acquisitions
Business Acquisition [Line Items]
Goodwill	7,200,000
2013 Acquisitions
Business Acquisition [Line Items]
Goodwill		38,900,000
Series of Individually Immaterial Business Acquisitions
Business Acquisition [Line Items]
Purchase price allocation, identifiable intangible assets		9,500,000
Goodwill		10,100,000
Contingent consideration		4,700,000
Purchase price allocation, deferred tax liability		1,200,000
2012 Acquisitions
Business Acquisition [Line Items]
Goodwill			$ 28,400,000