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Summary of Significant Accounting Policies - Concentration of Risk (Details) - customer | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Revenue | Customer concentration risk |  |  |
Concentration of Risk |  |  |
Number of customers | 2 | 2 | 1
Revenue | Customer concentration risk | Ten largest customers |  |  |
Concentration of Risk |  |  |
Number of customers | 10 | 10 | 10
Percentage of concentration risk | 76.90% | 73.30% | 70.20%
Revenue | Customer concentration risk | First customer |  |  |
Concentration of Risk |  |  |
Percentage of concentration risk | 20.10% | 24.90% | 17.00%
Revenue | Customer concentration risk | Second customer |  |  |
Concentration of Risk |  |  |
Percentage of concentration risk | 12.10% | 13.10% |
Consolidated accounts receivable | Credit concentration risk |  |  |
Concentration of Risk |  |  |
Number of customers | 2 | 3 |
Consolidated accounts receivable | Credit concentration risk | First customer |  |  |
Concentration of Risk |  |  |
Percentage of concentration risk | 21.90% | 19.80% |
Consolidated accounts receivable | Credit concentration risk | Second customer |  |  |
Concentration of Risk |  |  |
Percentage of concentration risk | 11.50% | 11.80% |
Consolidated accounts receivable | Credit concentration risk | Third customer |  |  |
Concentration of Risk |  |  |
Percentage of concentration risk |  | 10.60% |

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- Definition 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 -URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13531-108611 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6404-108592 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13537-108611 + Details
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