Lease Obligations (Schedule of Future Minimum Lease Commitments) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Finance Leases
2021, remaining nine months	$ 115,400
2022	115,300
2023	61,700
2024	22,500
2025	2,100
Thereafter	0
Total minimum lease payments	317,000
Less amounts representing interest	(14,200)
Total lease obligations, net of interest	302,800
Less current portion	142,200
Long-term portion of lease obligations, net of interest	160,600
Operating Leases
2021, remaining nine months	63,100
2022	51,500
2023	27,300
2024	17,300
2025	10,800
Thereafter	19,800
Total minimum lease payments	189,800
Less amounts representing interest	(13,600)
Total lease obligations, net of interest	176,200
Less current portion	69,184	$ 72,481
Long-term portion of lease obligations, net of interest	$ 106,971	$ 116,506