Sales Contracts (Schedule of Sales Contracts) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Original fair value, Assets	$ 97,196	$ 97,196
Original fair value, Liabilities	31,742	31,742
Accumulated amortization, Asset	(95,104)	(84,760)
Accumulated amortization, Liabilities	(30,783)	(29,979)
Total, Assets	2,092	12,436
Total, Liabilities	959	1,763
Net Total	1,133	10,673
Liabilities	562	934
Other current Assets
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Assets	2,091	12,432
Other Current Liabilities
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Liabilities	562	934
Other Noncurrent Assets
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Assets	1	4
Other Noncurrent Liabilities
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Liabilities	$ 397	$ 829