Note 9 - Fair Value Measurement - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Total assets	$ 26,551	$ 71,995	$ 106,814
Interest rate swap	5,770	7,606	9,058
Total liabilities	5,770	7,606	9,058
Fair Value, Inputs, Level 1 [Member]
Total assets	25,001	71,995	106,216
Interest rate swap	0	0	0
Total liabilities	0	0	0
Fair Value, Inputs, Level 2 [Member]
Total assets	1,550	0	598
Interest rate swap	5,770	7,606	9,058
Total liabilities	5,770	7,606	9,058
Fair Value, Inputs, Level 3 [Member]
Total assets	0	0	0
Interest rate swap	0	0	0
Total liabilities	0	0	0
Money Market Funds [Member]
Cash equivalents	23,489	70,483	104,704
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents	23,489	70,483	104,704
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Cash equivalents	0	0	0
Money Market Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Cash equivalents	0	0	0
Restricted Cash [Member]
Other noncurrent assets	1,512	1,512	1,512
Restricted Cash [Member] | Fair Value, Inputs, Level 1 [Member]
Other noncurrent assets	1,512	1,512	1,512
Restricted Cash [Member] | Fair Value, Inputs, Level 2 [Member]
Other noncurrent assets	0	0	0
Restricted Cash [Member] | Fair Value, Inputs, Level 3 [Member]
Other noncurrent assets	0	$ 0	0
Derivative Financial Instruments, Assets [Member] | Commodity Contract [Member]
Commodity swap	1,550		598
Derivative Financial Instruments, Assets [Member] | Fair Value, Inputs, Level 1 [Member] | Commodity Contract [Member]
Commodity swap	0		0
Derivative Financial Instruments, Assets [Member] | Fair Value, Inputs, Level 2 [Member] | Commodity Contract [Member]
Commodity swap	1,550		598
Derivative Financial Instruments, Assets [Member] | Fair Value, Inputs, Level 3 [Member] | Commodity Contract [Member]
Commodity swap	$ 0		$ 0