Lines of Business (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Lines of Business
External revenue	$ 563,439	$ 499,453
Intercompany revenue	0	0
Total revenue	563,439	499,453
Direct costs	426,659	385,591
Gross profit	136,780	113,862
Selling, general and administrative expense	50,128	44,398
Operating income (loss) before sale or disposal of assets	86,652	69,464
Net loss (gain) on sale or disposal of assets	563	335
Operating income (loss)	86,089	69,129
Depreciation and amortization	21,289	19,797
Property and equipment additions	25,806	16,526
Total assets	2,663,675		$ 2,501,132
Operating Segments [Member] | Animal Hospital [Member]
Lines of Business
External revenue	458,623	393,026
Intercompany revenue	0	0
Total revenue	458,623	393,026
Direct costs	385,206	337,542
Gross profit	73,417	55,484
Selling, general and administrative expense	12,085	11,221
Operating income (loss) before sale or disposal of assets	61,332	44,263
Net loss (gain) on sale or disposal of assets	575	294
Operating income (loss)	60,757	43,969
Depreciation and amortization	17,573	16,072
Property and equipment additions	18,544	12,082
Total assets	2,377,077		2,186,959
Operating Segments [Member] | Laboratory [Member]
Lines of Business
External revenue	89,240	78,809
Intercompany revenue	17,487	15,163
Total revenue	106,727	93,972
Direct costs	50,011	45,990
Gross profit	56,716	47,982
Selling, general and administrative expense	10,296	8,865
Operating income (loss) before sale or disposal of assets	46,420	39,117
Net loss (gain) on sale or disposal of assets	0	6
Operating income (loss)	46,420	39,111
Depreciation and amortization	2,781	2,504
Property and equipment additions	4,652	3,216
Total assets	322,776		306,296
Operating Segments [Member] | All Other [Member]
Lines of Business
External revenue	14,454	26,533
Intercompany revenue	4,959	7,694
Total revenue	19,413	34,227
Direct costs	12,503	22,803
Gross profit	6,910	11,424
Selling, general and administrative expense	5,299	8,687
Operating income (loss) before sale or disposal of assets	1,611	2,737
Net loss (gain) on sale or disposal of assets	0	9
Operating income (loss)	1,611	2,728
Depreciation and amortization	883	1,152
Property and equipment additions	607	800
Total assets	69,589		73,491
Corporate, Non-Segment [Member]
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	22,448	15,625
Operating income (loss) before sale or disposal of assets	(22,448)	(15,625)
Net loss (gain) on sale or disposal of assets	(12)	26
Operating income (loss)	(22,436)	(15,651)
Depreciation and amortization	638	592
Property and equipment additions	2,851	1,064
Total assets	586,394		471,112
Eliminations [Member]
Lines of Business
External revenue	1,122	1,085
Intercompany revenue	(22,446)	(22,857)
Total revenue	(21,324)	(21,772)
Direct costs	(21,061)	(20,744)
Gross profit	(263)	(1,028)
Selling, general and administrative expense	0	0
Operating income (loss) before sale or disposal of assets	(263)	(1,028)
Net loss (gain) on sale or disposal of assets	0	0
Operating income (loss)	(263)	(1,028)
Depreciation and amortization	(586)	(523)
Property and equipment additions	(848)	$ (636)
Total assets	$ (692,161)		$ (536,726)