Note 11 - Fair Value of Financial Instruments and Fair Value Measurements - Quantitative Information About Recurring Level 3 Fair Value Measurement (Details) $ in Thousands		Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value		$ 318,032	$ 354,247
Fair Value, Measurements, Recurring [Member]
Fair Value		318,032	354,247
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value		6,630	7,298
US States and Political Subdivisions Debt Securities [Member]
Fair Value		3,972	4,493
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value		3,972	4,493
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value		$ 215	216
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Measurement Input, Discount Rate [Member]
Range Weighted Average	[1]
US Government Agencies Debt Securities [Member]
Fair Value		$ 311,178	346,723
US Government Agencies Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value		311,178	346,723
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value		$ 4,415	5,022
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Measurement Input, Discount Rate [Member]
Range Weighted Average	[1]
Corporate Debt Securities [Member]
Fair Value		$ 2,882	2,060
Corporate Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value		2,882	2,060
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value		$ 2,000	$ 2,060
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, 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.