Lease Obligations (Schedule of Future Minimum Lease Commitments) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Finance Leases
2021	$ 141,500
2022	101,500
2023	47,600
2024	11,700
2025	500
Thereafter	0
Total minimum lease payments	302,800
Less amounts representing interest	(14,300)
Total lease obligations, net of interest	288,500
Less current portion	(132,600)
Long-term portion of lease obligations, net of interest	155,900
Operating Leases
2021	82,800
2022	49,200
2023	25,400
2024	16,200
2025	10,400
Thereafter	20,000
Total minimum lease payments	204,000
Less amounts representing interest	(15,000)
Total lease obligations, net of interest	189,000
Less current portion	72,481	$ 81,561
Long-term operating lease liabilities	$ 116,506	$ 154,553