Lease Obligations (Schedule of Future Minimum Lease Commitments) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Finance Leases
2021, remaining six months	$ 81,900
2022	127,700
2023	74,200
2024	34,100
2025	7,400
Thereafter	1,000
Total minimum lease payments	326,300
Less amounts representing interest	(13,600)
Total lease obligations, net of interest	312,700
Less current portion	145,400
Long-term portion of lease obligations, net of interest	$ 167,300
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	Current portion of long-term debt, including finance leases
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Long-term debt, including finance leases
Operating Leases
2021, remaining six months	$ 54,000
2022	72,600
2023	42,900
2024	30,000
2025	18,600
Thereafter	24,900
Total minimum lease payments	243,000
Less amounts representing interest	(14,600)
Total lease obligations, net of interest	228,400
Less current portion	85,573	$ 72,481
Long-term portion of lease obligations, net of interest	$ 142,777	$ 116,506