Lines of Business (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Lines of Business
External revenue	$ 678,251	$ 563,439
Intercompany revenue	0	0
Total revenue	678,251	563,439
Direct costs	523,783	426,659
Gross profit	154,468	136,780
Selling, general and administrative expense	58,401	50,128
Operating income (loss) before sale or disposal of assets	96,067	86,652
Net loss (gain) on sale or disposal of assets	250	563
Operating income (loss)	95,817	86,089
Depreciation and amortization	30,401	21,289
Property and equipment additions	28,919	25,806
Total assets	3,460,678		$ 3,373,328
Operating Segments | Animal Hospital
Lines of Business
External revenue	568,181	458,623
Intercompany revenue	0	0
Total revenue	568,181	458,623
Direct costs	481,871	385,206
Gross profit	86,310	73,417
Selling, general and administrative expense	17,611	12,085
Operating income (loss) before sale or disposal of assets	68,699	61,332
Net loss (gain) on sale or disposal of assets	211	575
Operating income (loss)	68,488	60,757
Depreciation and amortization	26,658	17,573
Property and equipment additions	19,603	18,544
Total assets	3,229,297		3,137,177
Operating Segments | Laboratory
Lines of Business
External revenue	90,549	89,240
Intercompany revenue	20,599	17,487
Total revenue	111,148	106,727
Direct costs	51,555	50,011
Gross profit	59,593	56,716
Selling, general and administrative expense	9,906	10,296
Operating income (loss) before sale or disposal of assets	49,687	46,420
Net loss (gain) on sale or disposal of assets	38	0
Operating income (loss)	49,649	46,420
Depreciation and amortization	2,908	2,781
Property and equipment additions	7,640	4,652
Total assets	344,355		331,484
Operating Segments | All Other
Lines of Business
External revenue	18,639	14,454
Intercompany revenue	3,930	4,959
Total revenue	22,569	19,413
Direct costs	13,883	12,503
Gross profit	8,686	6,910
Selling, general and administrative expense	6,640	5,299
Operating income (loss) before sale or disposal of assets	2,046	1,611
Net loss (gain) on sale or disposal of assets	1	0
Operating income (loss)	2,045	1,611
Depreciation and amortization	829	883
Property and equipment additions	1,118	607
Total assets	74,078		74,752
Corporate
Lines of Business
External revenue	0	0
Intercompany revenue	0	0
Total revenue	0	0
Direct costs	0	0
Gross profit	0	0
Selling, general and administrative expense	24,244	22,448
Operating income (loss) before sale or disposal of assets	(24,244)	(22,448)
Net loss (gain) on sale or disposal of assets	0	(12)
Operating income (loss)	(24,244)	(22,436)
Depreciation and amortization	692	638
Property and equipment additions	1,621	2,851
Total assets	1,587,845		1,502,150
Eliminations
Lines of Business
External revenue	882	1,122
Intercompany revenue	(24,529)	(22,446)
Total revenue	(23,647)	(21,324)
Direct costs	(23,526)	(21,061)
Gross profit	(121)	(263)
Selling, general and administrative expense	0	0
Operating income (loss) before sale or disposal of assets	(121)	(263)
Net loss (gain) on sale or disposal of assets	0	0
Operating income (loss)	(121)	(263)
Depreciation and amortization	(686)	(586)
Property and equipment additions	(1,063)	$ (848)
Total assets	$ (1,774,897)		$ (1,672,235)