Lines of Business (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Lines of Business
External revenue	$ 743,132	$ 653,489	$ 1,421,383	$ 1,216,928
Intercompany revenue	0	0	0	0
Total revenue	743,132	653,489	1,421,383	1,216,928
Direct costs	558,039	489,541	1,081,822	916,200
Gross profit	185,093	163,948	339,561	300,728
Selling, general and administrative expense	59,776	48,190	118,177	98,318
Operating income (loss) before sale or disposal of assets	125,317	115,758	221,384	202,410
Net (gain) loss on sale or disposal of assets	230	(271)	480	292
Operating income (loss)	125,087	116,029	220,904	202,118
Depreciation and amortization	30,519	25,689	60,920	46,978
Property and equipment additions	25,719	33,008	54,638	58,814
Total assets	3,537,725		3,537,725		$ 3,373,328
Operating Segments | Animal Hospital
Lines of Business
External revenue	628,798	540,376	1,196,979	998,999
Intercompany revenue	0	0	0	0
Total revenue	628,798	540,376	1,196,979	998,999
Direct costs	516,239	445,697	998,110	830,903
Gross profit	112,559	94,679	198,869	168,096
Selling, general and administrative expense	16,745	14,277	34,356	26,362
Operating income (loss) before sale or disposal of assets	95,814	80,402	164,513	141,734
Net (gain) loss on sale or disposal of assets	225	(132)	436	443
Operating income (loss)	95,589	80,534	164,077	141,291
Depreciation and amortization	26,486	21,875	53,144	39,448
Property and equipment additions	19,965	25,486	39,568	44,030
Total assets	3,282,596		3,282,596		3,137,177
Operating Segments | Laboratory
Lines of Business
External revenue	95,269	93,265	185,818	182,505
Intercompany revenue	21,932	18,795	42,531	36,282
Total revenue	117,201	112,060	228,349	218,787
Direct costs	53,777	51,513	105,332	101,524
Gross profit	63,424	60,547	123,017	117,263
Selling, general and administrative expense	9,975	9,702	19,881	19,998
Operating income (loss) before sale or disposal of assets	53,449	50,845	103,136	97,265
Net (gain) loss on sale or disposal of assets	3	(35)	41	(35)
Operating income (loss)	53,446	50,880	103,095	97,300
Depreciation and amortization	3,173	2,839	6,081	5,620
Property and equipment additions	5,042	4,750	12,682	9,402
Total assets	350,220		350,220		331,484
Operating Segments | All Other
Lines of Business
External revenue	18,225	18,656	36,864	33,110
Intercompany revenue	4,308	4,741	8,238	9,700
Total revenue	22,533	23,397	45,102	42,810
Direct costs	13,177	14,480	27,060	26,983
Gross profit	9,356	8,917	18,042	15,827
Selling, general and administrative expense	7,512	6,022	14,152	11,321
Operating income (loss) before sale or disposal of assets	1,844	2,895	3,890	4,506
Net (gain) loss on sale or disposal of assets	2	3	3	3
Operating income (loss)	1,842	2,892	3,887	4,503
Depreciation and amortization	849	904	1,678	1,787
Property and equipment additions	359	1,485	1,477	2,092
Total assets	74,838		74,838		74,752
Corporate
Lines of Business
External revenue	0	0	0	0
Intercompany revenue	0	0	0	0
Total revenue	0	0	0	0
Direct costs	0	0	0	0
Gross profit	0	0	0	0
Selling, general and administrative expense	25,544	18,189	49,788	40,637
Operating income (loss) before sale or disposal of assets	(25,544)	(18,189)	(49,788)	(40,637)
Net (gain) loss on sale or disposal of assets	0	(107)	0	(119)
Operating income (loss)	(25,544)	(18,082)	(49,788)	(40,518)
Depreciation and amortization	715	668	1,407	1,306
Property and equipment additions	1,259	2,336	2,880	5,187
Total assets	1,970,760		1,970,760		1,502,150
Eliminations
Lines of Business
External revenue	840	1,192	1,722	2,314
Intercompany revenue	(26,240)	(23,536)	(50,769)	(45,982)
Total revenue	(25,400)	(22,344)	(49,047)	(43,668)
Direct costs	(25,154)	(22,149)	(48,680)	(43,210)
Gross profit	(246)	(195)	(367)	(458)
Selling, general and administrative expense	0	0	0	0
Operating income (loss) before sale or disposal of assets	(246)	(195)	(367)	(458)
Net (gain) loss on sale or disposal of assets	0	0	0	0
Operating income (loss)	(246)	(195)	(367)	(458)
Depreciation and amortization	(704)	(597)	(1,390)	(1,183)
Property and equipment additions	(906)	$ (1,049)	(1,969)	$ (1,897)
Total assets	$ (2,140,689)		$ (2,140,689)		$ (1,672,235)