Note 20 - Quarterly Financial Information - Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Interest and loan fee income	$ 34,204	$ 33,145	$ 33,163	$ 33,324	$ 33,209	$ 33,468	$ 33,727	$ 33,647	$ 33,888	$ 34,299	$ 34,425	$ 33,917	$ 133,836	$ 134,051	$ 136,529
Net interest income	33,733	32,672	32,687	32,844	32,709	32,945	33,186	33,095	33,325	33,714	33,808	33,258	131,936	131,935	134,105
(Reversal of) provision for loan losses			(1,900)			(3,200)							(1,900)	(3,200)	0
Noninterest income	20,300	12,548	12,123	11,657	11,545	11,598	11,702	11,729	11,305	11,993	12,269	12,300	56,628	46,574	47,867
Noninterest expense	30,167	24,114	24,396	24,615	24,577	26,088	25,229	25,858	25,504	26,173	26,896	26,727	103,292	101,752	105,300
Income before taxes and equity in undistributed income of subsidiaries	23,866	21,106	22,314	19,886	19,677	21,655	19,659	18,966	19,126	19,534	19,181	18,831	87,172	79,957	76,672
Net income	$ 4,160	$ 15,017	$ 15,799	$ 15,049	$ 14,453	$ 15,628	$ 14,546	$ 14,226	$ 14,578	$ 14,857	$ 14,761	$ 14,557	$ 50,025	$ 58,853	$ 58,753
Basic earnings (in dollars per share)	$ 0.16	$ 0.57	$ 0.60	$ 0.58	$ 0.56	$ 0.61	$ 0.57	$ 0.56	$ 0.57	$ 0.58	$ 0.58	$ 0.57	$ 1.90	$ 2.30	$ 2.30
Diluted earnings (in dollars per share)	0.16	0.57	0.60	0.57	0.56	0.61	0.57	0.56	0.57	0.58	0.58	0.57	1.89	2.29	2.30
Dividends paid (in dollars per share)	0.40	0.39	0.39	0.39	0.39	0.39	0.39	0.39	0.39	0.38	0.38	0.38	1.57	1.56	1.53
Share price, common stock (in dollars per share)	59.55												59.55
Minimum [Member]
Share price, common stock (in dollars per share)	53.96	49.54	51.31	54.12	48.20	46.61	45.86	40.72	42.96	43	42.70	40.68	53.96	48.20	42.96
Maximum [Member]
Share price, common stock (in dollars per share)	$ 63.03	$ 59.54	$ 57.78	$ 64.07	$ 65.34	$ 50.96	$ 51.53	$ 49.63	$ 49.64	$ 51.90	$ 51.69	$ 48.44	$ 63.03	$ 65.34	$ 49.64