Fair Value Measurements Assets and Liabilities Measured on a Non-Recurring Basis (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Valuation adjustments to investments in joint venture arrangements	$ 0	$ 0	$ 0
Fair Value, Significant Unobservable Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjusted basis of inventory	3,823	12,921	11,885
Total losses	7,681	3,992	3,638
Initial basis of inventory	$ 11,504	$ 16,913	$ 15,523