Note 2 - Securities - Securities in a Continuous Loss Position (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Securities available for sale, continuous unrealized loss position, less than 12 months	$ 105,956,000	$ 60,173,000
Securities available for sale, continuous unrealized loss position, less than 12 months, unrealized loss	2,627,000	733,000
Securities available for sale, continuous unrealized loss position, 12 months or more	190,163,000	192,348,000
Securities available for sale, continuous unrealized loss position, 12 months or more, unrealized loss	11,412,000	6,826,000
Securities available for sale, continuous unrealized loss position	296,119,000	252,521,000
Securities available for sale, continuous unrealized loss position, unrealized loss	14,039,000	7,559,000
US Government Agencies Debt Securities [Member]
Securities available for sale, continuous unrealized loss position, less than 12 months	44,594,000	35,677,000
Securities available for sale, continuous unrealized loss position, less than 12 months, unrealized loss	1,464,000	434,000
Securities available for sale, continuous unrealized loss position, 12 months or more	126,600,000	115,374,000
Securities available for sale, continuous unrealized loss position, 12 months or more, unrealized loss	9,793,000	5,918,000
Securities available for sale, continuous unrealized loss position	171,194,000	151,051,000
Securities available for sale, continuous unrealized loss position, unrealized loss	11,257,000	6,352,000
Collateralized Mortgage Backed Securities [Member]
Securities available for sale, continuous unrealized loss position, less than 12 months	14,519,000	10,179,000
Securities available for sale, continuous unrealized loss position, less than 12 months, unrealized loss	479,000	156,000
Securities available for sale, continuous unrealized loss position, 12 months or more	18,867,000	21,084,000
Securities available for sale, continuous unrealized loss position, 12 months or more, unrealized loss	552,000	354,000
Securities available for sale, continuous unrealized loss position	33,386,000	31,263,000
Securities available for sale, continuous unrealized loss position, unrealized loss	1,031,000	510,000
Municipal General Obligation Bonds [Member]
Securities available for sale, continuous unrealized loss position, less than 12 months	43,826,000	12,807,000
Securities available for sale, continuous unrealized loss position, less than 12 months, unrealized loss	615,000	114,000
Securities available for sale, continuous unrealized loss position, 12 months or more	43,537,000	54,703,000
Securities available for sale, continuous unrealized loss position, 12 months or more, unrealized loss	1,010,000	524,000
Securities available for sale, continuous unrealized loss position	87,363,000	67,510,000
Securities available for sale, continuous unrealized loss position, unrealized loss	1,625,000	638,000
Municipal Revenue Bonds [Member]
Securities available for sale, continuous unrealized loss position, less than 12 months	1,499,000	0
Securities available for sale, continuous unrealized loss position, less than 12 months, unrealized loss	16,000	0
Securities available for sale, continuous unrealized loss position, 12 months or more	1,159,000	1,187,000
Securities available for sale, continuous unrealized loss position, 12 months or more, unrealized loss	57,000	30,000
Securities available for sale, continuous unrealized loss position	2,658,000	1,187,000
Securities available for sale, continuous unrealized loss position, unrealized loss	73,000	30,000
Other Debt and Equity Securities [Member]
Securities available for sale, continuous unrealized loss position, less than 12 months	1,518,000	1,510,000
Securities available for sale, continuous unrealized loss position, less than 12 months, unrealized loss	53,000	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	1,518,000	1,510,000
Securities available for sale, continuous unrealized loss position, unrealized loss	$ 53,000	$ 29,000