Leases - Operating and Finance Lease Right-of-use-assets and Related Lease Obligations (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Leases
Operating lease assets	$ 4,542	$ 5,849
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Operating lease assets
Finance lease assets	$ 20,544	21,880
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Finance lease assets
Total leased assets	$ 25,086	27,729
Operating liabilities current	$ 2,573	$ 3,144
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Other Liabilities, Current	Other Liabilities, Current
Finance liabilities current	$ 756	$ 399
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	Other Liabilities, Current
Operating liabilities noncurrent	$ 1,949	$ 2,553
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Other Liabilities, Noncurrent	Other Liabilities, Noncurrent
Finance liabilities noncurrent	$ 47,393	$ 48,149
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Finance liabilities noncurrent
Total lease liabilities	$ 52,671	54,245
Operating Lease, Liability, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent us-gaap:OtherLiabilitiesNoncurrent
Finance lease, accumulated depreciation	$ 48,400	47,400
Prepaid financing costs	$ 700	$ 700