Summary of Significant Accounting Policies - Concentration of Credit Risk (Details 3) - Sales Revenue, Goods, Net - Customer Concentration Risk	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cummins, Inc. [Member]
Summary of Significant Accounting Policies
Concentration of risk, percentage	13.40%	9.90%	9.30%
Honda Motor Co.
Summary of Significant Accounting Policies
Concentration of risk, percentage	11.20%	10.70%	10.70%
Toyota Motor Corporation
Summary of Significant Accounting Policies
Concentration of risk, percentage	10.20%	10.40%	10.10%