Note 8 - Income Taxes - Components of Income Tax Expense (Benefit) from Operations (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	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, 2018	Dec. 31, 2017	Dec. 31, 2016
Current income tax expense
Federal													$ 11,567	$ 12,622	$ 14,270
State													732	546	698
Total current income tax expense													12,299	13,168	14,968
Deferred income tax expense (benefit)
Federal													(52)	3,783	192
State													(44)	(4)	36
Total deferred income tax expense (benefit)													(96)	3,779	228
Change in valuation allowance													(172)	192	48
Total income tax expense	$ 2,265	$ 3,555	$ 3,159	$ 3,052	$ 5,542	$ 4,096	$ 4,359	$ 3,142	$ 4,009	$ 3,883	$ 3,676	$ 3,676	$ 12,031	$ 17,139	$ 15,244