Note 3 - Securities - Securities in a Continuous Loss Position (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Securities available for sale, continuous unrealized loss position, less than 12 months	$ 60,173,000	$ 183,125,000
Securities available for sale, continuous unrealized loss position, less than 12 months, unrealized loss	733,000	8,655,000
Securities available for sale, continuous unrealized loss position, 12 months or more	192,348,000	70,085,000
Securities available for sale, continuous unrealized loss position, 12 months or more, unrealized loss	6,826,000	774,000
Securities available for sale, continuous unrealized loss position	252,521,000	253,210,000
Securities available for sale, continuous unrealized loss position, unrealized loss	7,559,000	9,429,000
US Government Agencies Debt Securities [Member]
Securities available for sale, continuous unrealized loss position, less than 12 months	35,677,000	110,160,000
Securities available for sale, continuous unrealized loss position, less than 12 months, unrealized loss	434,000	7,172,000
Securities available for sale, continuous unrealized loss position, 12 months or more	115,374,000	5,073,000
Securities available for sale, continuous unrealized loss position, 12 months or more, unrealized loss	5,918,000	165,000
Securities available for sale, continuous unrealized loss position	151,051,000	115,233,000
Securities available for sale, continuous unrealized loss position, unrealized loss	6,352,000	7,337,000
Collateralized Mortgage Backed Securities [Member]
Securities available for sale, continuous unrealized loss position, less than 12 months	10,179,000	3,670,000
Securities available for sale, continuous unrealized loss position, less than 12 months, unrealized loss	156,000	4,000
Securities available for sale, continuous unrealized loss position, 12 months or more	21,084,000	37,072,000
Securities available for sale, continuous unrealized loss position, 12 months or more, unrealized loss	354,000	419,000
Securities available for sale, continuous unrealized loss position	31,263,000	40,742,000
Securities available for sale, continuous unrealized loss position, unrealized loss	510,000	423,000
Municipal General Obligation Bonds [Member]
Securities available for sale, continuous unrealized loss position, less than 12 months	12,807,000	65,895,000
Securities available for sale, continuous unrealized loss position, less than 12 months, unrealized loss	114,000	1,360,000
Securities available for sale, continuous unrealized loss position, 12 months or more	54,703,000	27,734,000
Securities available for sale, continuous unrealized loss position, 12 months or more, unrealized loss	524,000	189,000
Securities available for sale, continuous unrealized loss position	67,510,000	93,629,000
Securities available for sale, continuous unrealized loss position, unrealized loss	638,000	1,549,000
Municipal Revenue Bonds [Member]
Securities available for sale, continuous unrealized loss position, less than 12 months	0	1,921,000
Securities available for sale, continuous unrealized loss position, less than 12 months, unrealized loss	0	90,000
Securities available for sale, continuous unrealized loss position, 12 months or more	1,187,000	206,000
Securities available for sale, continuous unrealized loss position, 12 months or more, unrealized loss	30,000	1,000
Securities available for sale, continuous unrealized loss position	1,187,000	2,127,000
Securities available for sale, continuous unrealized loss position, unrealized loss	30,000	91,000
Other Debt and Equity Securities [Member]
Securities available for sale, continuous unrealized loss position, less than 12 months	1,510,000	1,479,000
Securities available for sale, continuous unrealized loss position, less than 12 months, unrealized loss	29,000	29,000
Securities available for sale, continuous unrealized loss position, 12 months or more	0	0
Securities available for sale, continuous unrealized loss position, 12 months or more, unrealized loss	0	0
Securities available for sale, continuous unrealized loss position	1,510,000	1,479,000
Securities available for sale, continuous unrealized loss position, unrealized loss	$ 29,000	$ 29,000