Note 3 - Investment Securities - Amortized Cost and Estimated Market Value of Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investment securities available for sale, amortized cost, 1 year or less	$ 202,597	$ 262,418
Investment securities available for sale, fair value, 1 year or less	202,637	261,976
Investment securities held to maturity, amortized cost, 1 year or less	81,436	86,172
Investment securities held to maturity, fair value, 1 year or less	81,589	86,148
Investment securities available for sale, amortized cost, over 1 to 5 years	1,329,781	1,438,849
Investment securities available for sale, fair value, over 1 to 5 years	1,342,419	1,414,020
Investment securities held to maturity, amortized cost, over 1 to 5 years	177,675	214,137
Investment securities held to maturity, fair value, over 1 to 5 years	180,218	213,829
Investment securities available for sale, amortized cost, over 5 to 10 years	193,678	85,817
Investment securities available for sale, fair value, over 5 to 10 years	199,098	85,877
Investment securities held to maturity, amortized cost, over 5 to 10 years	202,280	232,544
Investment securities held to maturity, fair value, over 5 to 10 years	207,471	233,515
Investment securities available for sale, amortized cost, over 10 years	38,443	38,672
Investment securities available for sale, fair value, over 10 years	39,066	36,970
Investment securities held to maturity, amortized cost, over 10 years	1,027	1,037
Investment securities held to maturity, fair value, over 10 years	1,068	1,074
Investment securities available for sale, amortized cost, subtotal	1,764,499	1,825,756
Investment securities available for sale, fair value, subtotal	1,783,220	1,798,843
Investment securities held to maturity, amortized cost, subtotal	462,418	533,890
Investment securities held to maturity, fair value, subtotal	470,346	534,566
Investment securities available for sale, amortized cost	992,768	885,697
Investment securities available for sale, fair value	992,679	855,827
Investment securities held to maturity, amortized cost	405,571	450,719
Investment securities held to maturity, fair value	402,630	436,879
Investment securities available for sale, amortized cost	2,757,267	2,711,453
Investment securities available for sale, fair value	2,775,899	2,654,670
Investment securities held to maturity, amortized cost	867,989	984,609
Investment securities held to maturity, fair value	$ 872,976	$ 971,445