Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net income	$ 59,894	$ 26,344	$ (22,829)	$ 28,069	$ 232,131	$ 156,940	$ 174,098
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization					106,021	95,030	80,529
Share-based compensation					16,764	15,618	14,983
Unrealized foreign exchange (gain) loss, net					26,655	(20,300)	10,200
Goodwill and Intangible Asset Impairment					7,049	34,056	0
Other (gains) losses					0	0	(19,449)
Deferred income taxes					2,425	(10,861)	850
Loss (income) from unconsolidated affiliates					117	(48)	0
Accretion of convertible debt discount and amortization of debt issuance costs					14,121	13,504	12,885
Changes in working capital, net of amounts acquired:
Income taxes payable, net					(13,317)	23,183	13,935
Trade accounts receivable					26,497	(198,089)	(87,732)
Prepaid expenses and other current assets					(29,066)	35,451	(75,261)
Trade accounts payable					45,562	3,840	9,705
Deferred revenue					9,349	3,724	9,426
Accrued expenses and other current liabilities					(37,595)	106,350	257,287
Changes in noncurrent assets and liabilities					(9,480)	27,878	(9,932)
Net cash provided by operating activities					397,233	286,276	391,524
Cash flows from investing activities:
Acquisitions, net of cash acquired					(12,854)	0	(55,915)
Purchases of property and equipment					(112,484)	(97,235)	(87,411)
Purchases of other long-term assets					(8,528)	(6,039)	(6,175)
Proceeds from sale of investment					0	0	11,900
Other, net					1,583	1,416	1,288
Net cash used in investing activities					(132,283)	(101,858)	(136,313)
Cash flows from financing activities:
Proceeds from issuance of shares					18,608	10,990	6,292
Repurchase of shares					(177,855)	(3,065)	(77,360)
Borrowings from revolving credit agreements					5,773,294	2,409,203	2,648,093
Repayments of revolving credit agreements					(5,560,089)	(2,566,621)	(2,495,632)
Repayments of long-term debt obligations					(52,199)	(8,907)	(7,031)
Repayments of capital lease obligations					(6,137)	(4,883)	(2,943)
Net borrowing (repayments of) from short-term debt obligations					9,472	1,853	6,750
Payments of Debt Issuance Costs					3,071		0	$ 0
Other, net					1	281	1,341
Net cash provided by (used in) financing activities					2,024	(161,149)	79,510
Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents					(36,540)	65,161	(25,463)
Increase in cash and cash equivalents and restricted cash					230,434	88,430	309,258
Cash and cash equivalents and restricted cash at beginning of period		$ 900,518		$ 812,088	900,518	812,088	502,830
Cash and cash equivalents and restricted cash at end of period	$ 1,130,952		$ 900,518		1,130,952	900,518	812,088	$ 502,830
Interest paid during the period					23,554	20,457	14,442
Income taxes paid during the period					84,382	48,644	43,178
Other Significant Noncash Transaction, Value of Consideration Received					$ 0	$ 0	$ 7,549