CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	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
Cash flows provided by (used in) operating activities:
Net income (loss)					$ (111,948,000)	$ 257,223,000	$ (40,621,000)
Income (Loss) from Continuing Operations, Including Portion Attributable to Noncontrolling Interest	$ (75,713,000)	$ (28,519,000)	$ 231,455,000	$ (9,683,000)	(111,948,000)	257,223,000	(40,621,000)
Share-based Compensation	1,534,000		7,480,000		18,416,000	24,350,000	21,003,000
Adjustments to reconcile income to cash provided by (used in) operating activities:
Depreciation					27,111,000	23,873,000	22,293,000
Amortization of intangibles					19,085,000	19,601,000	24,153,000
Amortization of deferred finance charges and debt discount					3,193,000	3,022,000	9,135,000
Asset Impairment Charges	56,732,000				80,057,000	4,415,000	48,524,000
Loss on redemption of Notes					0	0	26,873,000
Adjustment to deferred acquisition consideration					(374,000)	(4,819,000)	8,227,000
Deferred income taxes					23,573,000	(173,019,000)	(10,038,000)
(Earnings) losses of non-consolidated affiliates					(62,000)	(2,081,000)	309,000
Distributions from non-consolidated affiliates					963,000	3,672,000	7,402,000
Other and non-current assets and liabilities					392,000	(4,420,000)	13,527,000
Foreign exchange					20,795,000	(17,637,000)	(8,240,000)
Changes in working capital:
Accounts receivable					30,211,000	(50,030,000)	(16,752,000)
Expenditures billable to clients					11,223,000	(1,892,000)	(13,048,000)
Prepaid expenses and other current assets					(17,189,000)	6,569,000	(13,608,000)
Accounts payable, accruals and other current liabilities					(18,222,000)	13,398,000	(110,018,000)
Acquisition related payments					(29,141,000)	(42,790,000)	(44,914,000)
Cash in trust					(656,000)	(709,000)	219,000
Advance billings					(14,871,000)	14,548,000	11,397,000
Net cash provided by (used in) operating activities					17,280,000	71,786,000	(45,907,000)
Gain (Loss) on Disposition of Assets					(1,867,000)	(1,600,000)	(424,000)
Cash flows used in investing activities:
Capital expenditures					(20,264,000)	(32,958,000)	(29,432,000)
Increase (Decrease) in Deposit Assets					0	0	2,528,000
Payments for (Proceeds from) Investments					2,082,000	10,631,000	666,000
Acquisitions, net of cash acquired					(32,713,000)	0	2,531,000
Other investments					(499,000)	(2,229,000)	(3,835,000)
Net cash used in investing activities					(50,431,000)	(20,884,000)	(25,196,000)
Cash flows provided by (used in) financing activities:
Acquisition related payments					(32,172,000)	(57,083,000)	(90,779,000)
Distributions to noncontrolling interests					(13,419,000)	(8,865,000)	(7,772,000)
Proceeds from exercise of options						0	0
Payment of dividends					(196,000)	(284,000)	(32,918,000)
Repayment of long-term debt					(146,000)	(404,000)	(507,000)
Premium paid on redemption of Notes					0	0	(26,873,000)
Deferred financing costs					0	0	(21,569,000)
Purchase of shares					(776,000)	(1,758,000)	(3,350,000)
Net cash provided by (used in) financing activities					21,434,000	(32,599,000)	35,657,000
Effect of exchange rate changes on cash, cash equivalents, and cash held in trusts					77,000	(754,000)	2,128,000
Net increase (decrease) in cash, cash equivalents, and cash held in trusts including cash classified within assets held for sale					(11,640,000)	17,549,000	(33,318,000)
Net increase (decrease) in cash, cash equivalents, and cash held in trusts					(19,938,000)	17,549,000	(33,318,000)
Cash, cash equivalents, and cash held in trusts at beginning of year		$ 50,811,000		$ 33,262,000	50,811,000	33,262,000	66,580,000
Cash, cash equivalents, and cash held in trusts at end of year	30,873,000		50,811,000		30,873,000	50,811,000	33,262,000	$ 66,580,000
Supplemental disclosures:
Cash income taxes paid					3,836,000	8,099,000	2,895,000
Cash interest paid					64,012,000	62,895,000	64,671,000
Non-cash transactions:
Capital leases					95,000	670,000	265,000
Notes Receivable in exchange for shares of subsidiary						0	6,139,000	0
Dividends Payable	$ 196,000		$ 453,000		196,000	453,000	739,000
Stock Issued During Period, Value, Acquisitions							34,219,000
Leasehold improvements financed by landlord						0	0	$ 7,250,000
Six Point Five Zero Percentage Notes [Domain]
Cash flows provided by (used in) financing activities:
Proceeds from issuance of notes					0	0	900,000,000
6.75% Notes
Cash flows provided by (used in) financing activities:
Repayment of 6.75% Notes					0	0	(735,000,000)
Wells Fargo Capital Finance, Llc [Member] | Revolving Credit Facility [Member]
Cash flows provided by (used in) financing activities:
Repayments of Lines of Credit					(1,625,862,000)	(1,479,632,000)	(1,790,108,000)
Proceeds from Lines of Credit					1,694,005,000	1,425,207,000	1,844,533,000
Net decrease in cash, cash equivalents, and cash held in trusts classified within assets held for sale					(8,298,000)	0	0
Convertible preference shares
Cash flows provided by (used in) financing activities:
Proceeds from Issuance of Convertible Preferred Stock					0	95,000,000	0
Convertible Preferred Stock [Member]
Cash flows provided by (used in) financing activities:
Payments of Stock Issuance Costs					0	(4,780,000)	0
Common Class A
Non-cash transactions:
Redemption value adjustments						3,353,939
Common Stock
Non-cash transactions:
Redemption value adjustments					0
Common Stock | Common Class A
Non-cash transactions:
Redemption value adjustments						28,727,000	10,458,000
Forsman & Bodenfors AB [Domain] | Common Stock | Common Class A
Non-cash transactions:
Stock Issued During Period, Value, Acquisitions					$ 7,030,000	$ 0	$ 34,219,000