Note 14 - Financial Instruments - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivative assets	$ 20,129
Derivative liabilities	5,050	13,224
Fair Value, Measurements, Recurring [Member]
Derivative assets	20,129
Derivative liabilities	5,050	13,224
Fair Value, Measurements, Recurring [Member] | Fixed Price Derivative Contracts [Member]
Derivative assets	18,172
Derivative liabilities		13,208
Fair Value, Measurements, Recurring [Member] | Basis Swap [Member]
Derivative assets	1,957
Derivative liabilities	5,050	16
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative assets
Derivative liabilities
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Fixed Price Derivative Contracts [Member]
Derivative assets
Derivative liabilities
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Basis Swap [Member]
Derivative assets
Derivative liabilities
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative assets	18,172
Derivative liabilities		13,208
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Fixed Price Derivative Contracts [Member]
Derivative assets	18,172
Derivative liabilities		13,208
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Basis Swap [Member]
Derivative assets
Derivative liabilities
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative assets	1,957
Derivative liabilities	5,050	16
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Fixed Price Derivative Contracts [Member]
Derivative assets
Derivative liabilities
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Basis Swap [Member]
Derivative assets	1,957
Derivative liabilities	$ 5,050	$ 16