Leases (Assets and Liabilities Balance Related to Finance and Operating Leases) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Leases [Abstract]
Finance lease right-of-use assets	$ 6,618	$ 7,017
Operating lease right-of-use assets	4,802	4,216
Finance lease obligations, current portion	4,453	4,755
Finance lease obligations, non-current portion	184	322
Total finance lease obligations	4,637	5,077
Operating lease obligations, current portion	2,187	2,275
Operating lease obligations, non-current portion	2,644	2,219
Total operating lease obligations	$ 4,831	$ 4,494
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Property, plant and equipment, net of accumulated depreciation of $364,239 and $365,844	Property, plant and equipment, net of accumulated depreciation of $364,239 and $365,844
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Other long-term assets	Other long-term assets
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	Other accrued liabilities	Other accrued liabilities
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Other long-term liabilities	Other long-term liabilities
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Other accrued liabilities	Other accrued liabilities
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Other long-term liabilities	Other long-term liabilities