Note 11 - Fair Value of Financial Instruments and Fair Value Measurements - Quantitative Information About Recurring Level 3 Fair Value Measurement (Details) $ in Thousands		Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair Value		$ 409,839	$ 353,066
Fair Value, Recurring [Member]
Fair Value		409,839
Fair Value, Inputs, Level 3 [Member]
Fair Value			4,277
Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member]
Fair Value		3,434
US States and Political Subdivisions Debt Securities [Member]
Fair Value		3,574	3,989
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Recurring [Member]
Fair Value		3,574	3,989
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member]
Fair Value		$ 215	214
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | Measurement Input, Discount Rate [Member]
Range Weighted Average	[1]
US Government Agencies Debt Securities [Member]
Fair Value		$ 403,408	346,205
US Government Agencies Debt Securities [Member] | Fair Value, Recurring [Member]
Fair Value		403,408	346,205
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member]
Fair Value		$ 3,219	$ 4,063
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | Measurement Input, Discount Rate [Member]
Range Weighted Average	[1]

[1] The Company relies on a third-party pricing service to value its securities. The details of the unobservable inputs and other adjustments used by the third-party pricing service were not readily available to the Company.