Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net income	$ 217,403	$ 216,098	$ 140,683
Adjustments to reconcile net income to net cash provided by operating activities:
Impairment of goodwill and other long-lived assets	0	0	27,019
Gain on sale of business	0	(43,306)	0
Depreciation and amortization	102,942	81,688	79,427
Amortization of debt issue costs	1,652	1,741	1,391
Provision for uncollectible accounts	8,306	8,401	6,248
Debt retirement costs	1,600	0	1,709
Net loss (gain) on sale or disposal of assets	1,006	829	(1,152)
Share-based compensation	18,762	16,264	17,200
Excess tax benefits from share-based compensation	(10,711)	(11,089)	(6,241)
Deferred income taxes	18,339	56,722	8,853
Other	7,229	2,159	531
Changes in operating assets and liabilities:
Trade accounts receivable	(13,555)	(28,720)	(3,900)
Inventory, prepaid expenses and other assets	(22,784)	(9,716)	(22,897)
Accounts payable and other accrued liabilities	16,124	10,812	11,597
Accrued payroll and related liabilities	19,993	11,323	6,782
Income taxes	23,525	4,339	2,960
Net cash provided by operating activities	389,831	317,545	270,210
Cash flows from investing activities:
Business acquisitions, net of cash acquired	(697,673)	(151,586)	(147,507)
Investment in Vetstreet Inc.	0	(9,552)	0
Property and equipment additions	(122,946)	(95,234)	(72,948)
Proceeds from sale of assets	1,729	6,762	3,904
Proceeds from sale of business	0	48,000	0
Other	(9,351)	(2,042)	(2,691)
Net cash used in investing activities	(828,241)	(203,652)	(219,242)
Cash flows from financing activities:
Proceeds from the issuance of long-term obligations	1,255,000	0	600,000
Repayment of long-term obligations	(1,274,924)	(35,017)	(568,011)
Proceeds from revolving credit facility	555,000	97,000	135,000
Repayment of revolving credit facility	(95,000)	0	0
Payment of financing costs	(3,817)	0	(7,987)
Distributions to noncontrolling interest partners	(6,134)	(4,962)	(5,009)
Purchase of noncontrolling interests	(4,552)	(2,500)	(326)
Proceeds from issuance of common stock under stock incentive plans	3,965	2,683	2,859
Excess tax benefits from share-based compensation	10,711	11,089	6,241
Stock repurchases	(17,219)	(165,607)	(255,108)
Other	(1,714)	2,041	(1,424)
Net cash provided by (used in) financing activities	421,316	(95,273)	(93,765)
Effect of currency exchange rate changes on cash and cash equivalents	(385)	(1,115)	(849)
(Decrease) increase in cash and cash equivalents	(17,479)	17,505	(43,646)
Cash and cash equivalents at beginning of year	98,888	81,383	125,029
Cash and cash equivalents at end of year	81,409	98,888	81,383
Supplemental disclosures of cash flow information:
Interest paid	25,283	18,517	15,049
Income taxes paid	94,337	74,315	75,098
Supplemental schedule of non-cash investing and financing activities:
Capital lease additions	$ 7,090	$ 0	$ 0