Sales Contracts (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
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	(97,051)	(96,812)
Accumulated amortization, Liabilities	(31,098)	(30,924)
Total, Assets	145	384
Total, Liabilities	644	818
Net Total	(499)	(434)
Liabilities	425	570
Other current Assets
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Assets	145	384
Other Current Liabilities
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Liabilities	425	570
Other Noncurrent Assets
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Assets	0	0
Other Noncurrent Liabilities
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Liabilities	$ 219	$ 248