Lines of Business (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
External revenue									$ 2,516,863	$ 2,133,675	$ 1,918,483
Intercompany revenue									0	0	0
Total revenue	$ 643,081	$ 656,854	$ 653,489	$ 563,439	$ 533,720	$ 551,717	$ 548,785	$ 449,453	2,516,863	2,133,675	1,918,483
Direct costs									1,924,799	1,623,604	1,473,842
Gross profit	134,759	156,577	163,948	136,780	117,696	137,666	140,847	113,862	592,064	510,071	444,641
Selling, general and administrative expense									204,724	183,995	171,506
Operating income (loss) before charges									387,340	326,076	273,135
Impairment of goodwill and other long-lived assets									0	0	27,019
Net loss (gain) on sale or disposal of assets									1,006	829	(1,152)
Business interruption insurance gain						(4,500)			0	(4,523)	0
Operating income (loss)	77,218	$ 106,998	$ 116,029	$ 86,089	66,381	$ 97,079	$ 97,181	$ 69,129	386,334	329,770	247,268
Depreciation and amortization									102,942	81,688	79,427
Property and equipment additions									122,946	95,234	72,948
Total Assets	3,373,328				2,501,132				3,373,328	2,501,132	2,293,815
United States
Segment Reporting Information [Line Items]
Total revenue									2,315,225	1,957,842	1,760,946
Canada
Segment Reporting Information [Line Items]
Total revenue									201,638	175,833	157,537
Animal Hospital
Segment Reporting Information [Line Items]
Impairment of goodwill and other long-lived assets											0
Laboratory
Segment Reporting Information [Line Items]
Impairment of goodwill and other long-lived assets											0
Operating Segments | Animal Hospital
Segment Reporting Information [Line Items]
External revenue									2,091,780	1,697,870	1,514,878
Intercompany revenue									0	0	0
Total revenue									2,091,780	1,697,870	1,514,878
Direct costs									1,751,792	1,433,535	1,284,077
Gross profit									339,988	264,335	230,801
Selling, general and administrative expense									58,952	44,311	39,022
Operating income (loss) before charges									281,036	220,024	191,779
Net loss (gain) on sale or disposal of assets									1,290	(555)	1,473
Business interruption insurance gain										0
Operating income (loss)									279,746	220,579	190,306
Depreciation and amortization									87,818	66,129	61,031
Property and equipment additions									87,665	71,155	55,814
Total Assets	3,137,177				2,186,959				3,137,177	2,186,959	2,021,725
Operating Segments | Laboratory
Segment Reporting Information [Line Items]
External revenue									347,648	330,828	303,798
Intercompany revenue									74,653	63,072	56,598
Total revenue									422,301	393,900	360,396
Direct costs									203,143	192,198	184,588
Gross profit									219,158	201,702	175,808
Selling, general and administrative expense									39,979	38,075	33,550
Operating income (loss) before charges									179,179	163,627	142,258
Net loss (gain) on sale or disposal of assets									(49)	26	(88)
Business interruption insurance gain										0
Operating income (loss)									179,228	163,601	142,346
Depreciation and amortization									11,425	10,726	10,444
Property and equipment additions									26,093	17,418	8,306
Total Assets	331,484				306,296				331,484	306,296	258,550
Operating Segments | All Other
Segment Reporting Information [Line Items]
External revenue									73,311	100,843	96,053
Intercompany revenue									20,440	26,145	19,732
Total revenue									93,751	126,988	115,785
Direct costs									58,877	80,286	77,161
Gross profit									34,874	46,702	38,624
Selling, general and administrative expense									24,918	33,569	33,456
Operating income (loss) before charges									9,956	13,133	5,168
Impairment of goodwill and other long-lived assets											27,019
Net loss (gain) on sale or disposal of assets									3	82	(2,201)
Business interruption insurance gain										(4,523)
Operating income (loss)									9,953	17,574	(19,650)
Depreciation and amortization									3,472	4,640	6,940
Property and equipment additions									3,245	1,849	4,912
Total Assets	74,752				73,491				74,752	73,491	89,596
Corporate
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									80,875	68,040	65,478
Operating income (loss) before charges									(80,875)	(68,040)	(65,478)
Impairment of goodwill and other long-lived assets											0
Net loss (gain) on sale or disposal of assets									(238)	1,276	(336)
Business interruption insurance gain										0
Operating income (loss)									(80,637)	(69,316)	(65,142)
Depreciation and amortization									2,690	2,349	2,943
Property and equipment additions									10,558	8,319	6,649
Total Assets	1,502,150				471,112				1,502,150	471,112	232,209
Eliminations
Segment Reporting Information [Line Items]
External revenue									4,124	4,134	3,754
Intercompany revenue									(95,093)	(89,217)	(76,330)
Total revenue									(90,969)	(85,083)	(72,576)
Direct costs									(89,013)	(82,415)	(71,984)
Gross profit									(1,956)	(2,668)	(592)
Selling, general and administrative expense									0	0	0
Operating income (loss) before charges									(1,956)	(2,668)	(592)
Impairment of goodwill and other long-lived assets											0
Net loss (gain) on sale or disposal of assets									0	0	0
Business interruption insurance gain										0
Operating income (loss)									(1,956)	(2,668)	(592)
Depreciation and amortization									(2,463)	(2,156)	(1,931)
Property and equipment additions									(4,615)	(3,507)	(2,733)
Total Assets	$ (1,672,235)				$ (536,726)				$ (1,672,235)	$ (536,726)	$ (308,265)