Lease Obligations (Schedule of Future Minimum Lease Commitments) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Finance Leases
2019, remaining nine months	$ 60,700
2020	71,000
2021	57,300
2022	28,100
2023	3,400
Thereafter	100
Less amounts representing interest	220,600
Less amounts representing interest	(14,900)
Total lease obligations, net of interest	205,700
Less current portion	69,900
Long-term portion of lease obligations, net of interest	135,800
Operating Leases
2019, remaining nine months	64,600
2020	64,700
2021	42,200
2022	26,900
2023	16,900
Thereafter	36,100
Total minimum lease payments	251,400
Less amounts representing interest	(24,200)
Total lease obligations, net of interest	227,200
Less current portion	73,279	$ 0
Long-term portion of lease obligations, net of interest	$ 153,850	$ 0