Note 8 - Loans - Aging of Loan Portfolio (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Loans past due	$ 118,257	$ 131,462
Non-accrual loans	77,191	40,523
Loans not past due	15,447,522	14,944,019
Loans	15,565,779	15,075,481
Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	28,443	64,249
Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	9,755	20,281
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	2,868	6,409
Commercial Portfolio Segment [Member]
Loans past due	41,811	60,425
Non-accrual loans	29,757	19,381
Loans not past due	2,806,189	2,718,319
Loans	2,848,000	2,778,744
Commercial Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	4,956	24,681
Commercial Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	7,098	9,954
Commercial Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	0	6,409
Real Estate Construction Portfolio Segment [Member]
Loans past due	25,992	17,179
Non-accrual loans	4,335	4,580
Loans not past due	649,120	562,685
Loans	675,112	579,864
Real Estate Construction Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	19,000	5,846
Real Estate Construction Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	2,657	6,753
Real Estate Construction Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	0	0
Commercial Real Estate Portfolio Segment [Member]
Loans past due	40,925	20,231
Non-accrual loans	33,782	9,928
Loans not past due	7,418,391	7,255,031
Loans	7,459,316	7,275,262
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	4,275	7,694
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	0	2,609
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	2,868	0
Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans past due	9,529	33,627
Non-accrual loans	9,317	6,634
Loans not past due	4,572,166	4,402,934
Loans	4,581,695	4,436,561
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	212	26,028
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	0	965
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	0	0
Installment and Other Loans [Member ]
Loans past due	0	0
Non-accrual loans	0	0
Loans not past due	1,656	5,050
Loans	1,656	5,050
Installment and Other Loans [Member ] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	0	0
Installment and Other Loans [Member ] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	0	0
Installment and Other Loans [Member ] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	$ 0	$ 0