Note 4 - Securities - Available-for-sale Securities (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Amortized cost	$ 571,936,000	$ 564,391,000
Unrealized Gains	2,264,000	3,954,000
Unrealized Losses	4,126,000	2,469,000
Fair value	570,074,000	565,876,000
US Treasury Securities [Member]
Amortized cost	74,997,000	79,999,000
Unrealized Gains	1,000	1,000
Unrealized Losses
Fair value	74,998,000	80,000,000
US Government-sponsored Enterprises Debt Securities [Member]
Amortized cost	268,784,000	251,190,000
Unrealized Gains	800,000	1,468,000
Unrealized Losses	1,494,000	765,000
Fair value	268,090,000	251,893,000
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Amortized cost	170,344,000	170,139,000
Unrealized Gains	735,000	1,143,000
Unrealized Losses	2,236,000	1,654,000
Fair value	168,843,000	169,628,000
US States and Political Subdivisions Debt Securities [Member]
Amortized cost	57,158,000	62,410,000
Unrealized Gains	682,000	1,342,000
Unrealized Losses	396,000	50,000
Fair value	57,444,000	63,702,000
Common Stock [Member]
Amortized cost	653,000	653,000
Unrealized Gains	46,000
Unrealized Losses
Fair value	$ 699,000	$ 653,000