Note 3 - Investment Securities (Details Textual) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jan. 01, 2020	Dec. 31, 2019
Equity Securities, FV-NI	$ 0	$ 1,797		$ 0
Unrealized Gain (Loss) on Securities		$ 50
Debt Securities, Held-to-maturity, Allowance for Credit Loss, Ending Balance	16		$ 0	0
High Risk Collateralized Mortgage Obligations	0			0
Other-than-temporary Impairment Loss, Debt Securities, Available-for-sale, Total	0
Pledged Financial Instruments, Not Separately Reported, Securities, Total	868,665			760,365
Debt Securities, Available-for-sale, Amortized Cost, Total	3,575,297			3,041,860
Debt Securities, Available-for-sale, Total	3,708,370			3,078,846
Debt Securities, Held-to-maturity, Nonaccrual	0
Debt Security, Corporate, US [Member]
Debt Securities, Available-for-sale, Amortized Cost, Total	2,170,503			1,805,479
Debt Securities, Available-for-sale, Total	2,269,411			$ 1,833,783
Debt Security, Corporate, US [Member] \| External Credit Rating, Non Investment Grade [Member]
Debt Securities, Available-for-sale, Amortized Cost, Total	15,000
Debt Securities, Available-for-sale, Total	14,700
Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Debt Securities, Held-to-maturity, Allowance for Credit Loss, Ending Balance	$ 16		$ 16

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Investments consist of high-risk collateralized mortgage obligations as defined by regulartory guidelines.

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