Note 3 - Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Held-to-maturity, amortized cost, one year or less	$ 50,829
Held-to-maturity, fair value, one year or less	50,838
Available-for-sale, amortized cost, one year or less	21,211
Available-for-sale, fair value, one year or less	21,145
Held-to-maturity, amortized cost, after one through five years	105,323
Held-to-maturity, fair value, after one through five years	105,823
Available-for-sale, amortized cost, after one through five years	81,147
Available-for-sale, fair value, after one through five years	80,388
Held-to-maturity, amortized cost, after five through ten years	99,399
Held-to-maturity, fair value, after five through ten years	100,902
Available-for-sale, amortized cost, after five through ten years	14,532
Available-for-sale, fair value, after five through ten years	14,427
Held-to-maturity, amortized cost, after ten years	133,510
Held-to-maturity, fair value, after ten years	137,740
Available-for-sale, amortized cost, after ten years	181,619
Available-for-sale, fair value, after ten years	178,182
Held-to-maturity, amortized cost, securities not due on a single maturity date	16,972
Held-to-maturity, fair value, securities not due on a single maturity date	16,837
Available-for-sale, amortized cost, securities not due on a single maturity date	973,111
Available-for-sale, fair value, securities not due on a single maturity date	959,698
Available-for-sale, amortized cost, other securities (no maturity)	62,195
Available-for-sale, fair value, other securities (no maturity)	63,580
Held-to-maturity, amortized cost, total	406,033	$ 462,096
Held-to-maturity, fair value, total	412,140	465,960
Available-for-sale, amortized cost, total	1,333,815	1,181,484
Available-for-sale, fair value, total	$ 1,317,420	$ 1,157,354