Note 11 - Fair Value of Financial Instruments and Fair Value Measurements - Quantitative Information About Recurring Level 3 Fair Value Measurement (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value	$ 357,889	$ 353,066
Available for Sale, at Fair Value	357,889	353,066
Fair Value, Recurring [Member]
Fair Value	357,889	353,066
Available for Sale, at Fair Value	357,889	353,066
Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member]
Fair Value	3,897	4,277
Available for Sale, at Fair Value	3,897	4,277
US States and Political Subdivisions Debt Securities [Member]
Fair Value	3,895	3,989
Available for Sale, at Fair Value	3,895	3,989
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Recurring [Member]
Fair Value	3,895	3,989
Available for Sale, at Fair Value	3,895	3,989
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member]
Fair Value	215	214
Available for Sale, at Fair Value	215	214
US Government Agencies Debt Securities [Member]
Fair Value	351,138	346,205
Available for Sale, at Fair Value	351,138	346,205
US Government Agencies Debt Securities [Member] | Fair Value, Recurring [Member]
Fair Value	351,138	346,205
Available for Sale, at Fair Value	351,138	346,205
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member]
Fair Value	3,682	4,063
Available for Sale, at Fair Value	$ 3,682	$ 4,063