Note 13 - Income Taxes - Income Tax Reconciliation (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, 2017	Dec. 31, 2016	Dec. 31, 2015
Taxes at statutory federal income tax rate, amount									$ 12,007	$ 8,504	$ 5,324
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent									35.00%	35.00%	35.00%
Graduated tax rates, amount										$ (243)	$ (94)
Graduated tax rates, percent										(1.00%)	(0.60%)
State income tax—net of federal benefits, amount									$ 1,142	$ 865	$ 484
State income tax—net of federal benefits, percent									3.30%	3.60%	3.20%
Valuation allowance—net, amount										$ (897)
Valuation allowance—net, percent										(3.70%)
Earnings on life insurance policies, amount									$ (994)	$ (549)	$ (515)
Earnings on life insurance policies, percent									(2.90%)	(2.30%)	(3.40%)
Nontaxable investments, amount									$ (1,260)	$ (677)	$ (619)
Nontaxable investments, percent									(3.70%)	(2.80%)	(4.10%)
Change in deferred tax asset related to tax reform, amount									$ 2,355
Change in deferred tax asset related to tax reform, percent									6.90%
Other—net, amount									$ (255)	$ (144)	$ 59
Other—net, percent									(0.70%)	(0.60%)	0.40%
Total, amount	$ 5,544	$ 2,072	$ 3,078	$ 2,300	$ 2,192	$ 2,384	$ 847	$ 1,436	$ 12,995	$ 6,859	$ 4,639
Total, percent									37.90%	28.20%	30.50%