Lease Obligations - Summary of lease-related assets and liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets
Operating lease right-of-use assets	$ 21,447	$ 22,291
Finance lease right-of-use assets	$ 0	390
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	Property and equipment, net of accumulated depreciation of $313,999 and $294,366, respectively
Total right-of-use lease assets	$ 21,447	22,681
Current
Operating lease obligations	7,451	6,612
Finance lease obligations	0	318
Total current obligations	7,451	6,930
Non-current
Operating lease obligations	14,015	15,730
Total non-current obligations	14,015	15,730
Total lease obligations	$ 21,466	$ 22,660