Lease Obligations (Schedule of Future Minimum Lease Commitments) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Finance Leases
2019, remaining six months	$ 48,600
2020	92,700
2021	79,000
2022	41,300
2023	7,600
Thereafter	300
Total minimum lease payments	269,500
Less amounts representing interest	(16,600)
Total lease obligations, net of interest	252,900
Less current portion	86,900
Long-term portion of lease obligations, net of interest	166,000
Operating Leases
2019, remaining six months	45,900
2020	75,800
2021	53,700
2022	32,300
2023	17,800
Thereafter	41,300
Total minimum lease payments	266,800
Less amounts representing interest	(20,500)
Total lease obligations, net of interest	246,300
Less current portion	77,629	$ 0
Long-term portion of lease obligations, net of interest	$ 168,698	$ 0