Note 3 - Investment Securities - Credit Quality (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Held to maturity	$ 638,297	$ 738,072
AAA/AA/A [Member]
Held to maturity	637,166
BBB [Member]
Held to maturity	101
BB/B [Member]
Held to maturity	1,030
Agency Residential MBS [Member]
Held to maturity	304,439	353,937
Agency Residential MBS [Member] | AAA/AA/A [Member]
Held to maturity	304,439
Agency Residential MBS [Member] | BBB [Member]
Held to maturity	0
Agency Residential MBS [Member] | BB/B [Member]
Held to maturity	0
Non-agency Residential MBS [Member]
Held to maturity	2,112	2,354
Non-agency Residential MBS [Member] | AAA/AA/A [Member]
Held to maturity	1,082
Non-agency Residential MBS [Member] | BBB [Member]
Held to maturity	0
Non-agency Residential MBS [Member] | BB/B [Member]
Held to maturity	1,030
US States and Political Subdivisions Debt Securities [Member]
Held to maturity	331,746	$ 381,781
US States and Political Subdivisions Debt Securities [Member] | AAA/AA/A [Member]
Held to maturity	331,645
US States and Political Subdivisions Debt Securities [Member] | BBB [Member]
Held to maturity	101
US States and Political Subdivisions Debt Securities [Member] | BB/B [Member]
Held to maturity	$ 0