Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net income	$ 10,812	$ 37,435	$ 65,885
Adjustment to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	64,507	68,675	63,808
Amortization of contract acquisition costs	1,678	659	900
Amortization of debt issuance costs	745	753	712
Imputed interest expense and fair value adjustments to contingent consideration	51	(4,523)	716
Provision for doubtful accounts	458	1,164	1,465
(Gain) loss on disposal of assets	3,694	(44)	(166)
Gain on sale of business	(908)	0	0
Impairment losses	5,322	32,050	8,100
Non-cash loss on assets held-for-sale	0	5,300	0
Deferred income taxes	16,777	(1,583)	9,317
Excess tax benefit from equity-based awards	(2,192)	(601)	(662)
Equity-based compensation expense	11,852	9,773	11,304
(Gain) loss on foreign currency derivatives	(681)	1,710	(1,469)
Changes in assets and liabilities, net of acquisitions:
Accounts receivable	(59,284)	(7,858)	(19,867)
Prepaids and other assets	(19,266)	(92)	(5,379)
Accounts payable and accrued expenses	18,968	(19,141)	16,189
Deferred revenue and other liabilities	60,619	(11,847)	(12,855)
Net cash provided by operating activities	113,152	111,830	137,998
Cash flows from investing activities
Proceeds from sale of long-live assets	39	114	202
Purchases of property, plant and equipment	(51,958)	(50,832)	(66,595)
Proceeds from sale of business	636	0	0
Investments in non-marketable equity investments	(1,384)	(3,179)	(9,000)
Acquisitions, net of cash acquired of zero and zero, respectively	(116,320)	(46,460)	(1,776)
Net cash used in investing activities	(168,987)	(100,357)	(77,169)
Cash flows from financing activities
Proceeds from line of credit	2,293,587	2,093,500	2,262,350
Payments on line of credit	(2,166,887)	(1,976,200)	(2,262,350)
Payments on other debt	(6,041)	(3,222)	(3,222)
Payments of contingent consideration and hold back payments to acquisitions	(1,409)	(9,467)	(11,883)
Dividends paid to shareholders	(21,531)	(18,262)	(17,423)
Payments to noncontrolling interest	(3,645)	(4,317)	(4,593)
Purchase of mandatorily redeemable noncontrolling interest	0	(4,105)	0
Proceeds from exercise of stock options	2,150	371	825
Tax payments related to issuance of restricted stock units	(5,397)	(3,933)	(4,248)
Excess tax benefit from equity-based awards	0	601	662
Payments of debt issuance costs	(918)	(1,888)	(35)
Purchase of treasury stock	(18,328)	(74,683)	(17,231)
Net cash provided by financing activities	71,581	(1,605)	(57,148)
Effect of exchange rate changes on cash and cash equivalents	3,427	(14,908)	(20,693)
Increase (decrease) in cash and cash equivalents	19,173	(5,040)	(17,012)
Cash and cash equivalents, beginning of period	55,264	60,304	77,316
Cash and cash equivalents, end of period	74,437	55,264	60,304
Supplemental disclosures
Cash paid for interest	11,727	6,976	6,052
Cash paid for income taxes	18,813	19,741	15,178
Non-cash investing and financing activities
Acquisition of long lived assets through capital leases	9,836	584	10,492
Acquisition of equipment through increase in accounts payable, net	97	(681)	386
Contract acquisition costs credited to accounts receivable	$ 0	$ 206	$ 1,055