Note 3 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Temporarily impaired securities, less than 12 months, fair value	$ 87,068	$ 295,295
Temporarily impaired securities, less than 12 months unrealized loss	1,065	7,232
Temporarily impaired securities, 12 months or more fair value	229,675	2,475
Temporarily impaired securities, 12 months or more unrealized loss	4,750	178
US Government Agencies Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	68,380	260,150
Temporarily impaired securities, less than 12 months unrealized loss	871	6,161
Temporarily impaired securities, 12 months or more fair value	225,738	996
Temporarily impaired securities, 12 months or more unrealized loss	4,623	4
US States and Political Subdivisions Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	18,688	31,257
Temporarily impaired securities, less than 12 months unrealized loss	194	934
Temporarily impaired securities, 12 months or more fair value	2,989	1,316
Temporarily impaired securities, 12 months or more unrealized loss	98	148
Corporate Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value		3,888
Temporarily impaired securities, less than 12 months unrealized loss		137
Temporarily impaired securities, 12 months or more fair value	948	0
Temporarily impaired securities, 12 months or more unrealized loss	$ 29	0
Other Securities [Member]
Temporarily impaired securities, less than 12 months, fair value		0
Temporarily impaired securities, less than 12 months unrealized loss		0
Temporarily impaired securities, 12 months or more fair value		163
Temporarily impaired securities, 12 months or more unrealized loss		$ 26