Condensed Consolidated Statement Of Cash Flows - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Cash flows from operating activities:
Net income	$ 18,871	$ 25,820
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization	42,618	39,323
Amortization of financing costs and discounts	2,852	2,853
Share-based compensation	6,420	3,614
Provision for doubtful accounts	4,134	2,640
Deferred income taxes	354	(2,361)
Changes in fair value of contingent consideration	4,100	400
Loss on equity securities	3,678	0	$ 0
Decrease (increase) in:
Accounts receivable	59,647	25,110
Prepaid expenses and other current assets	2,574	(291)
Other assets	2,132	162
(Decrease) increase in:
Accounts payable and accrued expenses	(45,144)	(53,566)
Income taxes payable	(1,721)	4,795
Deferred revenue	3,210	2,384
Liability for uncertain tax positions	933	573
Other long-term liabilities	(748)	(265)
Net cash provided by operating activities	103,910	51,191
Cash flows from investing activities:
Payments to Acquire Investments	(13,403)	(5)
Purchases of property and equipment	(13,165)	(9,660)
Acquisition of businesses, net of cash received	(80,223)	(3,563)
Purchases of intangible assets	(175)	(142)
Net cash used in investing activities	(106,966)	(13,370)
Cash flows from financing activities:
Proceeds from Lines of Credit	0	44,981
Repurchase and retirement of common stock	(611)	(314)
Issuance of common stock under employee stock purchase plan	64	67
Exercise of stock options	594	695
Dividends paid	(19,884)	(17,575)
Deferred payments for acquisitions	(189)	(2,299)
Proceeds from (Payments for) Other Financing Activities	(54)	(26)
Net cash (used in) provided by financing activities	(20,080)	25,529
Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	3,960	109
Net change in cash, cash equivalents and restricted cash	(19,176)	63,459
Cash, cash equivalents and restricted cash at beginning of period	350,945	123,950	123,950
Cash, cash equivalents and restricted cash at end of period	$ 331,769	$ 187,409	$ 350,945