Lines of Business (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
External revenue									$ 2,133,675	$ 1,918,483	$ 1,803,369
Intercompany revenue									0	0	0
Total revenue	$ 533,720	$ 551,717	$ 548,785	$ 449,453	$ 479,927	$ 499,577	$ 489,472	$ 449,507	2,133,675	1,918,483	1,803,369
Direct costs									1,623,604	1,473,842	1,393,989
Gross profit	117,696	137,666	140,847	113,862	99,018	123,757	120,415	101,451	510,071	444,641	409,380
Selling, general and administrative expense									183,995	171,506	157,911
Operating income (loss) before charges									326,076	273,135	251,469
Impairment of goodwill and other long-lived assets										27,019
Business interruption insurance gain		(4,500)							(4,523)	0	0
Net loss (gain) on sale or disposal of assets									829	(1,152)	2,455
Operating income (loss)	66,381	$ 97,079	$ 97,181	$ 69,129	52,654	$ 53,476	$ 79,906	$ 61,232	329,770	247,268	249,014
Depreciation and amortization									81,688	79,427	77,409
Property and equipment additions									95,234	72,948	73,270
Total Assets	2,507,265				2,301,689				$ 2,507,265	2,301,689	2,199,191
Disclosure on Geographic Areas, Description of Revenue from External Customers									*Revenues are determined based on the location of the customer.
United States
Segment Reporting Information [Line Items]
Total revenue									$ 1,957,842	1,760,946	1,671,170
Canada
Segment Reporting Information [Line Items]
Total revenue									175,833	157,537	132,199
Operating Segments [Member] | Animal Hospital [Member]
Segment Reporting Information [Line Items]
External revenue									1,697,870	1,514,878	1,417,908
Intercompany revenue									0	0	0
Total revenue									1,697,870	1,514,878	1,417,908
Direct costs									1,433,535	1,284,077	1,208,781
Gross profit									264,335	230,801	209,127
Selling, general and administrative expense									44,311	39,022	34,133
Operating income (loss) before charges									220,024	191,779	174,994
Impairment of goodwill and other long-lived assets										0
Net loss (gain) on sale or disposal of assets									(555)	1,473	2,853
Operating income (loss)									220,579	190,306	172,141
Depreciation and amortization									66,129	61,031	57,748
Property and equipment additions									71,155	55,814	54,116
Total Assets	2,186,959				2,021,725				2,186,959	2,021,725	1,844,235
Operating Segments [Member] | Laboratory [Member]
Segment Reporting Information [Line Items]
External revenue									330,828	303,798	290,583
Intercompany revenue									63,072	56,598	54,248
Total revenue									393,900	360,396	344,831
Direct costs									192,198	184,588	180,952
Gross profit									201,702	175,808	163,879
Selling, general and administrative expense									38,075	33,550	31,915
Operating income (loss) before charges									163,627	142,258	131,964
Impairment of goodwill and other long-lived assets										0
Business interruption insurance gain									0
Net loss (gain) on sale or disposal of assets									26	(88)	(77)
Operating income (loss)									163,601	142,346	132,041
Depreciation and amortization									10,726	10,444	10,251
Property and equipment additions									17,418	8,306	11,148
Total Assets	306,296				258,550				306,296	258,550	248,282
Operating Segments [Member] | All Other [Member]
Segment Reporting Information [Line Items]
External revenue									100,843	96,053	91,686
Intercompany revenue									26,145	19,732	21,054
Total revenue									126,988	115,785	112,740
Direct costs									80,286	77,161	74,139
Gross profit									46,702	38,624	38,601
Selling, general and administrative expense									33,569	33,456	32,941
Operating income (loss) before charges									13,133	5,168	5,660
Impairment of goodwill and other long-lived assets										27,019
Business interruption insurance gain									(4,523)
Net loss (gain) on sale or disposal of assets									82	(2,201)	7
Operating income (loss)									17,574	(19,650)	5,653
Depreciation and amortization									4,640	6,940	7,909
Property and equipment additions									1,849	4,912	4,288
Total Assets	73,491				89,596				73,491	89,596	96,245
Corporate [Member]
Segment Reporting Information [Line Items]
External revenue									0	0	0
Intercompany revenue									0	0	0
Total revenue									0	0	0
Direct costs									0	0	0
Gross profit									0	0	0
Selling, general and administrative expense									68,040	65,478	58,922
Operating income (loss) before charges									(68,040)	(65,478)	(58,922)
Impairment of goodwill and other long-lived assets										0
Business interruption insurance gain									0
Net loss (gain) on sale or disposal of assets									1,276	(336)	(328)
Operating income (loss)									(69,316)	(65,142)	(58,594)
Depreciation and amortization									2,349	2,943	3,284
Property and equipment additions									8,319	6,649	5,908
Total Assets	477,245				240,083				477,245	240,083	48,246
Eliminations [Member]
Segment Reporting Information [Line Items]
External revenue									4,134	3,754	3,192
Intercompany revenue									(89,217)	(76,330)	(75,302)
Total revenue									(85,083)	(72,576)	(72,110)
Direct costs									(82,415)	(71,984)	(69,883)
Gross profit									(2,668)	(592)	(2,227)
Selling, general and administrative expense									0	0	0
Operating income (loss) before charges									(2,668)	(592)	(2,227)
Impairment of goodwill and other long-lived assets										0
Business interruption insurance gain									0
Net loss (gain) on sale or disposal of assets									0	0	0
Operating income (loss)									(2,668)	(592)	(2,227)
Depreciation and amortization									(2,156)	(1,931)	(1,783)
Property and equipment additions									(3,507)	(2,733)	(2,190)
Total Assets	$ (536,726)				$ (308,265)				$ (536,726)	$ (308,265)	$ (37,817)