Note 8 - Income Taxes - Components of Income Tax Expense (Benefit) from Operations (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current tax expense
Federal													$ 12,622	$ 14,270	$ 15,478
State													546	698	608
Total current tax expense													13,168	14,968	16,086
Deferred tax expense (benefit)
Federal													3,783	192	748
State													(4)	36	54
Total deferred tax expense (benefit)													3,779	228	802
Change in valuation allowance													192	48	45
Total income tax expense	$ 5,542	$ 4,096	$ 4,359	$ 3,142	$ 4,009	$ 3,883	$ 3,676	$ 3,676	$ 4,177	$ 4,352	$ 4,151	$ 4,253	$ 17,139	$ 15,244	$ 16,933