Goodwill and Other Long-Lived Assets - Changes in Carrying Amount of Goodwill (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2017 USD ($)
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 2,294,479
Accumulated impairment losses, beginning balance	(130,057)
Subtotal, beginning balance	2,164,422
Goodwill acquired	109,090
Foreign translation adjustment	6,317
Other	(15,564)
Goodwill, ending balance	2,394,322
Accumulated impairment losses, ending balance	(130,057)
Subtotal, ending balance	2,264,265
Animal Hospital
Goodwill [Roll Forward]
Goodwill, beginning balance	2,047,894
Accumulated impairment losses, beginning balance	0
Subtotal, beginning balance	2,047,894
Goodwill acquired	109,090
Foreign translation adjustment	6,301
Other	(15,564)
Goodwill, ending balance	2,147,721
Accumulated impairment losses, ending balance	0
Subtotal, ending balance	2,147,721
Laboratory
Goodwill [Roll Forward]
Goodwill, beginning balance	101,283
Accumulated impairment losses, beginning balance	0
Subtotal, beginning balance	101,283
Goodwill acquired	0
Foreign translation adjustment	16
Other	0
Goodwill, ending balance	101,299
Accumulated impairment losses, ending balance	0
Subtotal, ending balance	101,299
All Other
Goodwill [Roll Forward]
Goodwill, beginning balance	145,302
Accumulated impairment losses, beginning balance	(130,057)
Subtotal, beginning balance	15,245
Goodwill acquired	0
Foreign translation adjustment	0
Other	0
Goodwill, ending balance	145,302
Accumulated impairment losses, ending balance	(130,057)
Subtotal, ending balance	$ 15,245