Note 9 - Commitments and Contingencies (Details Textual) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Leases, Rent Expense, Net, Total	$ 24,300,000	$ 15,197,000	$ 18,205,000
Office Space in Dickinson, North Dakota [Member]
Operating Leases, Rent Expense, Net, Total	$ 23,200	27,840	27,165
Office Space in Lusk, Wyoming [Member]
Operating Leases, Rent Expense, Net, Total		9,000	9,000
Office Space in Denver, Colorado [Member]
Operating Leases, Rent Expense, Net, Total		$ 13,837	$ 15,601