Acquisitions - Summary of Purchase Price and Allocation of Purchase Price (Details) - USD ($) $ in Thousands		8 Months Ended	12 Months Ended
	May 01, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Consideration:
Cash, net of cash acquired			$ 697,673	$ 151,586	$ 147,507
Assumed debt		$ 11,414	11,414	16,885	7,426
Earn-outs		9,200	9,200	3,500
Allocation of the Purchase Price:
Goodwill				640,877	125,705
Noncontrolling interest			$ (86,640)	(1,916)	(933)
Weighted average amortization period			5 years
Amount of goodwill deducted for income tax purpose		263,400	$ 263,400	100,100	62,500
Customer Relationships
Allocation of the Purchase Price:
Weighted average amortization period			5 years
Trademarks
Allocation of the Purchase Price:
Weighted average amortization period			8 years
Covenants not-to-compete
Allocation of the Purchase Price:
Weighted average amortization period			5 years
Client lists
Allocation of the Purchase Price:
Weighted average amortization period			3 years
Companion Animal Practices, North America
Consideration:
Cash	$ 352,829
Cash acquired	(3,405)
Cash, net of cash acquired	349,424
Fair value of total consideration transferred	350,424	5,400
Allocation of the Purchase Price:
Identifiable intangible assets	102,300
Fair value of assets acquired and liabilities assumed	$ 436,424
Weighted average amortization period	7 years
Amount of goodwill deducted for income tax purpose		270,300	$ 270,300
Companion Animal Practices, North America | Customer Relationships
Allocation of the Purchase Price:
Weighted average amortization period	7 years
Companion Animal Practices, North America | Trademarks
Allocation of the Purchase Price:
Weighted average amortization period	5 years
Companion Animal Practices, North America | Covenants not-to-compete
Allocation of the Purchase Price:
Weighted average amortization period	5 years
Animal Hospitals And Laboratory Acquisitions, Excluding CAPNA
Consideration:
Cash			351,404	147,379	122,804
Cash acquired			(6,530)	(258)	(1)
Cash, net of cash acquired			344,874	147,121	122,803
Holdbacks		9,479	9,479	5,040	3,000
Earn-outs		7,281	7,281	1,671	2,037
Fair value of total consideration transferred			373,048	170,717	135,266
Allocation of the Purchase Price:
Tangible assets		32,212	32,212	11,482	5,902
Identifiable intangible assets		47,121	47,121	41,216	22,964
Goodwill			302,957	124,198	110,234
Other liabilities assumed		(8,344)	(8,344)	(2,206)	(115)
Fair value of assets acquired and liabilities assumed		373,946	373,946	174,690	138,985
Noncontrolling interest			(2,015)	(2,675)	(1,705)
Notes receivable from shareholder for formation of noncontrolling interest		$ 1,375	1,375	(1,298)	0
Preferred stock issued			(258)	0	0
Fair value of pre-existing investment			$ 0	$ 0	$ (2,014)