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Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenue	$ 2,595,777	$ 1,758,439	$ 1,666,487
Gross revenue	2,595,777	2,402,321	$ 2,364,956
Top client
Disaggregation of Revenue [Line Items]
Revenue	352,335
Gross revenue		503,875
Clients 2-5
Disaggregation of Revenue [Line Items]
Revenue	671,723
Gross revenue		505,818
Clients 6-10
Disaggregation of Revenue [Line Items]
Revenue	385,741
Gross revenue		348,841
Clients 11-25
Disaggregation of Revenue [Line Items]
Revenue	461,351
Gross revenue		421,759
Other
Disaggregation of Revenue [Line Items]
Revenue	$ 724,627
Gross revenue		$ 622,028

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- Definition

Revenue from Contract with Customer, Excluding Assessed Tax, Gross

+ References

No definition available.

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- Definition

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+ References

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- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 32
-Paragraph 2A
-URI http://asc.fasb.org/extlink&oid=82859839&loc=SL82860478-203043

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82913847&loc=SL49130543-203045

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82913847&loc=SL49130539-203045

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=82913847&loc=SL49130545-203045

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