Note 7 - Financial Instruments - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Derivative assets	$ 70,808	$ 4,253
Derivative liabilities	800	11,687
Derivative liabilities	800	11,687
Fair Value, Recurring [Member]
Derivative assets	70,808	4,253
Derivative liabilities	800	11,687
Derivative liabilities	800	11,687
Fair Value, Recurring [Member] | Fixed Price Derivative Contracts [Member]
Derivative assets	69,533	4,253
Derivative liabilities		5,583
Derivative liabilities		5,583
Fair Value, Recurring [Member] | Basis Swap [Member]
Derivative assets	1,275
Derivative liabilities	800	6,104
Derivative liabilities	800	6,104
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative assets
Derivative liabilities
Derivative liabilities
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Fixed Price Derivative Contracts [Member]
Derivative assets
Derivative liabilities
Derivative liabilities
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Basis Swap [Member]
Derivative assets
Derivative liabilities
Derivative liabilities
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative assets	69,533	4,253
Derivative liabilities		5,583
Derivative liabilities		5,583
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Fixed Price Derivative Contracts [Member]
Derivative assets	69,533	4,253
Derivative liabilities		5,583
Derivative liabilities		5,583
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Basis Swap [Member]
Derivative assets
Derivative liabilities
Derivative liabilities
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative assets	1,275
Derivative liabilities	800	6,104
Derivative liabilities	800	6,104
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Fixed Price Derivative Contracts [Member]
Derivative assets
Derivative liabilities
Derivative liabilities
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Basis Swap [Member]
Derivative assets	1,275
Derivative liabilities	800	6,104
Derivative liabilities	$ 800	$ 6,104