Note M - Leases (Details Textual) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Operating Lease, Payments	$ 100		$ 400
Property, Plant and Equipment, Net, Ending Balance	419,045		419,045		$ 387,275
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment, Ending Balance	196,931		196,931		$ 175,887
Depreciation, Total	13,403	$ 14,173	41,936	$ 40,974
Trucks under Operating Lease [Member]
Property, Plant and Equipment, Net, Ending Balance	53,400		53,400
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment, Ending Balance	24,600		24,600
Depreciation, Total	1,400		4,900
Dock Space under Operating Lease [Member]
Property, Plant and Equipment, Net, Ending Balance	13,700		13,700
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment, Ending Balance	1,200		1,200
Depreciation, Total	100		200
Leases without Early Cancellation Options [Member] | Other Noncurrent Assets [Member]
Operating Lease, Right-of-Use Asset	1,700		1,700
Operating Lease, Liability, Total	$ 1,700		$ 1,700
Trucks under Operating Lease [Member]
Lessor, Operating Lease, Term of Contract (Year)	5 years		5 years
Minimum [Member] | Shop and Office Leases with Early Cancellation Options [Member]
Lessee, Operating Lease, Term of Contract (Year)	1 year		1 year
Minimum [Member] | Leases without Early Cancellation Options [Member]
Lessee, Operating Lease, Term of Contract (Year)	3 years		3 years
Maximum [Member] | Shop and Office Leases with Early Cancellation Options [Member]
Lessee, Operating Lease, Term of Contract (Year)	5 years		5 years
Maximum [Member] | Leases without Early Cancellation Options [Member]
Lessee, Operating Lease, Term of Contract (Year)	5 years		5 years