Summary of significant accounting policies - Concentration of Risk (Details) - Customer Concentration Risk - Revenue from Contract with Customer	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Huawei Technologies Co., Ltd
Concentration Risk [Line Items]
Percentage of concentration of credit risk	41.00%	46.00%	40.00%
Ciena Corporation
Concentration Risk [Line Items]
Percentage of concentration of credit risk	29.00%	24.00%	16.00%
Top Five Customers
Concentration Risk [Line Items]
Percentage of concentration of credit risk	84.00%	87.00%	78.00%

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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

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