Note 3 - Investment Securities (Details) $ in Millions	1 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($)	Dec. 31, 2018 USD ($) state security item	Dec. 31, 2017 USD ($) item
Number of securities | security		611
Available-for-sale, unrealized loss positions, qualitative disclosure, number of positions | security		350
Aggregate losses of securities (as a percent)		1.90%
Available-for-sale, unrealized loss positions, qualitative disclosure, number of positions, greater than or equal to one year | security		188
Cost of investment | $	$ 0.0
Gain on sale | $	$ 4.0
Security Owned and Pledged as Collateral, Fair Value | $		$ 100.9	$ 78.6
Number of states holding investments | state		50
Number Of Charters Owning Municipal Securities | item		5
Municipal Bonds | General Obligation Bonds
Number of issuers | item		110	131
Other investments | $		$ 86.4	$ 108.0
Number of states holding investments | item		26	26
Municipal Bonds | General Obligation Bonds | Minimum
Other investments | $		$ 5.0	$ 5.0
Municipal Bonds | General Obligation Bonds | Aggregate Fair Value Exceeding 5 Million [Member]
Number of states holding investments | item		6	6
Municipal Bonds | Revenue Bonds
Number of issuers | item		160	145
Other investments | $		$ 371.9	$ 337.3
Number of states holding investments | item		19	16
Municipal Bonds | Revenue Bonds | Minimum
Other investments | $		$ 5.0	$ 5.0
Municipal Bonds | Revenue Bonds | Aggregate Fair Value Exceeding 5 Million [Member]
Number of states holding investments | item		7	7