Note 3 - Investment Securities (Details Textual) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Securities Continuous Unrealized Loss Position, Fair Value			$ 1,200,000,000
Securities Continuous Unrealized Loss Position Fair Value Percent of Portfolio			69.70%
Available-for-sale Securities Pledged as Collateral	$ 985,800,000		$ 985,800,000		$ 915,200,000
Available-for-sale Securities, Gross Realized Gains	3,000	$ 315,000	2,300,000	$ 4,400,000
Available-for-sale Securities, Gross Realized Losses	$ 0	$ 0	$ 5,000	$ 0