Note 7 - Investment Securities - Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Investment securities available for sale, amortized cost	$ 1,330,232	$ 1,443,730
Investment securities available for sale, gross unrealized gains	27,776	9,852
Investment securities available for sale, gross unrealized losses	2,835	1,740
Investment securities available for sale, Estimated fair value	1,355,173	1,451,842
Securities available-for-sale (amortized cost of $1,330,232 at March 31, 2020 and $1,443,730 at December 31, 2019)	1,355,173	1,451,842
Securities available-for-sale, amortized cost	1,330,232	1,443,730
US Government Corporations and Agencies Securities [Member]
Investment securities available for sale, amortized cost	110,650	90,452
Investment securities available for sale, gross unrealized gains	151	663
Investment securities available for sale, gross unrealized losses	391	319
Investment securities available for sale, Estimated fair value	110,410	90,796
Securities available-for-sale (amortized cost of $1,330,232 at March 31, 2020 and $1,443,730 at December 31, 2019)	110,410	90,796
Securities available-for-sale, amortized cost	110,650	90,452
US Treasury Securities [Member]
Investment securities available for sale, amortized cost		74,926
Investment securities available for sale, gross unrealized gains		10
Investment securities available for sale, gross unrealized losses		0
Investment securities available for sale, Estimated fair value		74,936
Securities available-for-sale (amortized cost of $1,330,232 at March 31, 2020 and $1,443,730 at December 31, 2019)		74,936
Securities available-for-sale, amortized cost		74,926
US Government-sponsored Enterprises Debt Securities [Member]
Investment securities available for sale, amortized cost	175,000	225,000
Investment securities available for sale, gross unrealized gains	45	0
Investment securities available for sale, gross unrealized losses	0	557
Investment securities available for sale, Estimated fair value	175,045	224,443
Securities available-for-sale (amortized cost of $1,330,232 at March 31, 2020 and $1,443,730 at December 31, 2019)	175,045	224,443
Securities available-for-sale, amortized cost	175,000	225,000
Collateralized Mortgage Backed Securities [Member]
Investment securities available for sale, amortized cost	883,097	880,040
Investment securities available for sale, gross unrealized gains	27,570	8,574
Investment securities available for sale, gross unrealized losses	663	824
Investment securities available for sale, Estimated fair value	910,004	887,790
Securities available-for-sale (amortized cost of $1,330,232 at March 31, 2020 and $1,443,730 at December 31, 2019)	910,004	887,790
Securities available-for-sale, amortized cost	883,097	880,040
Collateralized Mortgage Obligations [Member]
Investment securities available for sale, amortized cost	459	569
Investment securities available for sale, gross unrealized gains	0	0
Investment securities available for sale, gross unrealized losses	11	17
Investment securities available for sale, Estimated fair value	448	552
Securities available-for-sale (amortized cost of $1,330,232 at March 31, 2020 and $1,443,730 at December 31, 2019)	448	552
Securities available-for-sale, amortized cost	459	569
Corporate Debt Securities [Member]
Investment securities available for sale, amortized cost	161,026	172,743
Investment securities available for sale, gross unrealized gains	10	605
Investment securities available for sale, gross unrealized losses	1,770	23
Investment securities available for sale, Estimated fair value	159,266	173,325
Securities available-for-sale (amortized cost of $1,330,232 at March 31, 2020 and $1,443,730 at December 31, 2019)	159,266	173,325
Securities available-for-sale, amortized cost	$ 161,026	$ 172,743

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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CollateralizedMortgageObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: