Note 2 - Investment Securities - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Continuous unrealized loss, less than 12 months, fair value	$ 139,240	$ 177,689
Continuous unrealized loss, less than 12 months, gross unrealized losses	(2,022)	(3,490)
Continuous unrealized loss, greater than 12 months, fair value	126,175	107,482
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(4,113)	(4,213)
Continuous unrealized loss, fair value	265,415	285,171
Continuous unrealized loss, gross unrealized losses	(6,135)	(7,703)
US Government Agencies Debt Securities [Member]
Continuous unrealized loss, less than 12 months, fair value	136,825	174,201
Continuous unrealized loss, less than 12 months, gross unrealized losses	(1,973)	(3,460)
Continuous unrealized loss, greater than 12 months, fair value	126,175	107,482
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(4,113)	(4,213)
Continuous unrealized loss, fair value	263,000	281,683
Continuous unrealized loss, gross unrealized losses	(6,086)	(7,673)
US States and Political Subdivisions Debt Securities [Member]
Continuous unrealized loss, less than 12 months, fair value	1,423	3,488
Continuous unrealized loss, less than 12 months, gross unrealized losses	(25)	(30)
Continuous unrealized loss, greater than 12 months, fair value
Continuous unrealized loss, greater than 12 months, gross unrealized losses
Continuous unrealized loss, fair value	1,423	3,488
Continuous unrealized loss, gross unrealized losses	(25)	$ (30)
Asset-backed Securities [Member]
Continuous unrealized loss, less than 12 months, fair value	992
Continuous unrealized loss, less than 12 months, gross unrealized losses	(24)
Continuous unrealized loss, greater than 12 months, fair value
Continuous unrealized loss, greater than 12 months, gross unrealized losses
Continuous unrealized loss, fair value	992
Continuous unrealized loss, gross unrealized losses	$ (24)