Note 3 - Investment Securities - Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investment securities available for sale, amortized cost	$ 1,443,730	$ 1,267,731
Investment securities available for sale, gross unrealized gains	9,852	1,778
Investment securities available for sale, gross unrealized losses	1,740	27,000
Investment securities available for sale, Estimated fair value	1,451,842	1,242,509
Securities available-for-sale (amortized cost of $1,443,730 in 2019 and $1,267,731 in 2018)	1,451,842	1,242,509
US Treasury Securities [Member]
Investment securities available for sale, amortized cost	74,926	124,801
Investment securities available for sale, gross unrealized gains	10	0
Investment securities available for sale, gross unrealized losses	0	50
Investment securities available for sale, Estimated fair value	74,936	124,751
Securities available-for-sale (amortized cost of $1,443,730 in 2019 and $1,267,731 in 2018)	74,936	124,751
US Government Corporations and Agencies Securities [Member]
Investment securities available for sale, amortized cost	90,452	6,066
Investment securities available for sale, gross unrealized gains	663	0
Investment securities available for sale, gross unrealized losses	319	195
Investment securities available for sale, Estimated fair value	90,796	5,871
Securities available-for-sale (amortized cost of $1,443,730 in 2019 and $1,267,731 in 2018)	90,796	5,871
US Government-sponsored Enterprises Debt Securities [Member]
Investment securities available for sale, amortized cost	225,000	400,000
Investment securities available for sale, gross unrealized gains	0	0
Investment securities available for sale, gross unrealized losses	557	11,638
Investment securities available for sale, Estimated fair value	224,443	388,362
Securities available-for-sale (amortized cost of $1,443,730 in 2019 and $1,267,731 in 2018)	224,443	388,362
Collateralized Mortgage Backed Securities [Member]
Investment securities available for sale, amortized cost	880,040	670,874
Investment securities available for sale, gross unrealized gains	8,574	960
Investment securities available for sale, gross unrealized losses	824	15,089
Investment securities available for sale, Estimated fair value	887,790	656,745
Securities available-for-sale (amortized cost of $1,443,730 in 2019 and $1,267,731 in 2018)	887,790	656,745
Collateralized Mortgage Obligations [Member]
Investment securities available for sale, amortized cost	569	1,005
Investment securities available for sale, gross unrealized gains	0	0
Investment securities available for sale, gross unrealized losses	17	28
Investment securities available for sale, Estimated fair value	552	977
Securities available-for-sale (amortized cost of $1,443,730 in 2019 and $1,267,731 in 2018)	552	977
Corporate Debt Securities [Member]
Investment securities available for sale, amortized cost	172,743	64,985
Investment securities available for sale, gross unrealized gains	605	818
Investment securities available for sale, gross unrealized losses	23	0
Investment securities available for sale, Estimated fair value	173,325	65,803
Securities available-for-sale (amortized cost of $1,443,730 in 2019 and $1,267,731 in 2018)	$ 173,325	$ 65,803

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-SubTopic 10
-Section 50
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on 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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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