Note 4 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Temporarily impaired securities, less than 12 months, fair value	$ 110,999	$ 87,068
Temporarily impaired securities, less than 12 months unrealized loss	2,891	1,065
Temporarily impaired securities, 12 months or more fair value	230,059	229,675
Temporarily impaired securities, 12 months or more unrealized loss	6,369	4,750
US Government Agencies Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	76,276	68,380
Temporarily impaired securities, less than 12 months unrealized loss	1,987	871
Temporarily impaired securities, 12 months or more fair value	226,261	225,738
Temporarily impaired securities, 12 months or more unrealized loss	6,103	4,623
US States and Political Subdivisions Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	29,753	18,688
Temporarily impaired securities, less than 12 months unrealized loss	843	194
Temporarily impaired securities, 12 months or more fair value	2,889	2,989
Temporarily impaired securities, 12 months or more unrealized loss	198	98
Corporate Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	4,970
Temporarily impaired securities, less than 12 months unrealized loss	61
Temporarily impaired securities, 12 months or more fair value	909	948
Temporarily impaired securities, 12 months or more unrealized loss	$ 68	$ 29