Summary of Significant Accounting Policies - Concentration of Credit Risk (Details 3) - Sales Revenue, Goods, Net - Customer Concentration Risk	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cummins, Inc. [Member]
Summary of Significant Accounting Policies
Concentration of risk, percentage	15.20%	13.40%	9.90%
Honda Motor Co.
Summary of Significant Accounting Policies
Concentration of risk, percentage	10.50%	11.20%	10.70%
Toyota Motor Corporation
Summary of Significant Accounting Policies
Concentration of risk, percentage	10.50%	10.20%	10.40%

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
-Topic 825
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

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