Note 3 - Investment Securities - Amortized Cost and Estimated Market Value of Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investment securities available for sale, amortized cost, 1 year or less	$ 124,211	$ 193,337
Investment securities available for sale, fair value, 1 year or less	123,868	193,385
Investment securities held to maturity, amortized cost, 1 year or less	77,266	50,295
Investment securities held to maturity, fair value, 1 year or less	77,884	51,105
Investment securities available for sale, amortized cost, over 1 to 5 years	1,192,306	1,031,807
Investment securities available for sale, fair value, over 1 to 5 years	1,162,929	1,023,047
Investment securities held to maturity, amortized cost, over 1 to 5 years	239,877	269,050
Investment securities held to maturity, fair value, over 1 to 5 years	239,547	269,471
Investment securities available for sale, amortized cost, over 5 to 10 years	180,999	159,266
Investment securities available for sale, fair value, over 5 to 10 years	179,109	160,042
Investment securities held to maturity, amortized cost, over 5 to 10 years	256,872	277,170
Investment securities held to maturity, fair value, over 5 to 10 years	257,158	281,546
Investment securities available for sale, amortized cost, over 10 years	39,837	46,065
Investment securities available for sale, fair value, over 10 years	37,617	45,154
Investment securities held to maturity, amortized cost, over 10 years	2,940	2,963
Investment securities held to maturity, fair value, over 10 years	3,016	3,074
Investment securities available for sale, amortized cost, subtotal	1,537,353	1,430,475
Investment securities available for sale, fair value, subtotal	1,503,523	1,421,628
Investment securities held to maturity, amortized cost, subtotal	576,955	599,478
Investment securities held to maturity, fair value, subtotal	577,605	605,196
Investment securities available for sale, amortized cost	896,685	790,076
Investment securities available for sale, fair value	859,671	770,079
Investment securities held to maturity, amortized cost	499,501	559,386
Investment securities held to maturity, fair value	480,968	550,146
Investment securities available for sale, amortized cost	2,434,038	2,220,551
Investment securities available for sale, fair value	2,363,194	2,191,707
Investment securities held to maturity, amortized cost	1,076,456	1,158,864
Investment securities held to maturity, fair value	$ 1,058,573	$ 1,155,342