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Summary of Significant Accounting Policies - Concentration of Credit Risk (Details) - Net Revenues - Customer Concentration	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Concentration of Credit Risk
Percentage of concentration risk	73.00%	74.00%	82.00%
Minimum
Concentration of Credit Risk
Duration of customer contracts (in years)	3 years
Maximum
Concentration of Credit Risk
Duration of customer contracts (in years)	5 years

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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