Note 4 - Loans - Aging of Loan Portfolio (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Loans past due	$ 216,542	$ 131,462
Non-accrual loans	67,684	40,523
Loans not past due	15,427,854	14,944,019
Loans	15,644,396	15,075,481
Loans, amortized cost, greater than 90 days and accruing	4,982	6,409
Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	118,402	64,249
Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	25,474	20,281
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	4,982	6,409
Commercial Portfolio Segment [Member]
Loans past due	81,817	60,425
Non-accrual loans	23,087	19,381
Loans not past due	2,755,016	2,718,319
Loans	2,836,833	2,778,744
Loans, amortized cost, greater than 90 days and accruing	2,947	6,409
Commercial Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	52,601	24,681
Commercial Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	3,182	9,954
Commercial Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	2,947	6,409
Real Estate Construction Portfolio Segment [Member]
Loans past due	10,543	17,179
Non-accrual loans	4,286	4,580
Loans not past due	668,949	562,685
Loans	679,492	579,864
Loans, amortized cost, greater than 90 days and accruing	0	0
Real Estate Construction Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	6,257	5,846
Real Estate Construction Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	0	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	99,005	20,231
Non-accrual loans	33,715	9,928
Loans not past due	7,456,022	7,255,031
Loans	7,555,027	7,275,262
Loans, amortized cost, greater than 90 days and accruing	2,035	0
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	45,186	7,694
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	18,069	2,609
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	2,035	0
Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans past due	25,134	33,627
Non-accrual loans	6,596	6,634
Loans not past due	4,544,810	4,402,934
Loans	4,569,944	4,436,561
Loans, amortized cost, greater than 90 days and accruing	0	0
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	14,315	26,028
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	4,223	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	43	0
Non-accrual loans	0	0
Loans not past due	3,057	5,050
Loans	3,100	5,050
Loans, amortized cost, greater than 90 days and accruing	0	0
Installment and Other Loans [Member ] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	43	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