Acquisitions - Acquisition and Disposal Activity (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2016 USD ($) Laboratory Hospital	Mar. 31, 2015 USD ($) Laboratory Hospital	Dec. 31, 2015 USD ($)
Consideration:
Cash, net of cash acquired	$ 160,385	$ 33,652
Earn-outs	6,700		$ 3,500
Allocation of the Purchase Price:
Goodwill	1,656,389		$ 1,517,650
Animal Hospitals And Laboratory Acquisitions [Member]
Consideration:
Cash, net of cash acquired	157,325	31,850
Assumed debt	1,361	4,446
Holdbacks	3,508	1,722
Earn-outs	3,437	0
Fair value of total consideration transferred	165,631	38,018
Allocation of the Purchase Price:
Tangible assets	17,541	764
Identifiable intangible assets	18,844	2,838
Goodwill	129,246	34,563
Other liabilities assumed	0	(147)
Fair value of assets acquired and liabilities assumed	$ 165,631	38,018
Weighted average amortization period	5 years
Goodwill recorded for acquisitions, fully deductible for income tax purposes	$ 129,200	$ 30,400
Animal Hospitals And Laboratory Acquisitions [Member] | Customer Relationships [Member]
Allocation of the Purchase Price:
Weighted average amortization period	5 years
Animal Hospitals And Laboratory Acquisitions [Member] | Trademarks [Member]
Allocation of the Purchase Price:
Weighted average amortization period	2 years
Animal Hospitals And Laboratory Acquisitions [Member] | Covenants [Member]
Allocation of the Purchase Price:
Weighted average amortization period	5 years
Animal Hospital [Member]
Business Acquisition Summary Of Acquired Properties [Line Items]
Acquisitions | Hospital	24	11
Acquisitions, merged | Hospital	(1)	(2)
Sold, closed or merged | Hospital	(2)	(2)
Net increase | Hospital	21	7
Laboratories Acquisitions [Member]
Business Acquisition Summary Of Acquired Properties [Line Items]
Acquisitions | Laboratory	0	1
Acquisitions, merged | Laboratory	0	(1)
Net increase | Laboratory	0	0