Note 7 - Financial Instruments - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Derivative assets	$ 13,965	$ 20,129
Derivative liabilities	7,314	5,050
Fair Value, Recurring [Member]
Derivative assets	13,965	20,129
Derivative liabilities	7,314	5,050
Fair Value, Recurring [Member] | Fixed Price Derivative Contracts [Member]
Derivative assets	13,965	18,172
Derivative liabilities	335
Fair Value, Recurring [Member] | Basis Differential Swap [Member]
Derivative assets		1,957
Derivative liabilities	6,979	5,050
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative assets
Derivative liabilities
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Fixed Price Derivative Contracts [Member]
Derivative assets
Derivative liabilities
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Basis Differential Swap [Member]
Derivative assets
Derivative liabilities
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative assets	13,965	18,172
Derivative liabilities	335
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Fixed Price Derivative Contracts [Member]
Derivative assets	13,965	18,172
Derivative liabilities	335
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Basis Differential Swap [Member]
Derivative assets
Derivative liabilities
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative assets		1,957
Derivative liabilities	6,979	5,050
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Fixed Price Derivative Contracts [Member]
Derivative assets
Derivative liabilities
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Basis Differential Swap [Member]
Derivative assets		1,957
Derivative liabilities	$ 6,979	$ 5,050