Leases - Operating and Finance Lease Right-of-use-assets and Related Lease Obligations (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Leases
Operating lease assets	$ 6,431
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Operating lease assets
Finance lease assets	$ 22,563
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Finance lease assets
Total leased assets	$ 28,994
Operating liabilities current	$ 3,389
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Other Liabilities, Current
Finance liabilities current	$ 108
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	Other Liabilities, Current
Operating liabilities noncurrent	$ 3,020
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Other Liabilities, Noncurrent
Finance liabilities noncurrent	$ 48,441	$ 47,757
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Finance liabilities noncurrent
Total lease liabilities	$ 54,958
Finance lease, accumulated depreciation	47,000
Prepaid financing costs	$ 800