Note 8 - Loans - Aging of Loan Portfolio (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Loans past due	$ 140,650	$ 216,542
Non-accrual loans	94,446	67,684
Loans not past due	15,511,198	15,427,854
Loans	15,651,848	15,644,396
Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	41,802	118,402
Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	2,264	25,474
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	2,138	4,982
Commercial Portfolio Segment [Member]
Loans past due	55,721	81,817
Non-accrual loans	38,351	23,087
Loans not past due	2,834,972	2,755,016
Loans	2,890,693	2,836,833
Commercial Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	13,845	52,601
Commercial Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	1,645	3,182
Commercial Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	1,880	2,947
Real Estate Construction Portfolio Segment [Member]
Loans past due	4,189	10,543
Non-accrual loans	4,189	4,286
Loans not past due	673,627	668,949
Loans	677,816	679,492
Real Estate Construction Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	0	6,257
Real Estate Construction Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	0	0
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	50,552	99,005
Non-accrual loans	43,361	33,715
Loans not past due	7,498,970	7,456,022
Loans	7,549,522	7,555,027
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	6,659	45,186
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	274	18,069
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	258	2,035
Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans past due	30,188	25,134
Non-accrual loans	8,545	6,596
Loans not past due	4,500,333	4,544,810
Loans	4,530,521	4,569,944
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	21,298	14,315
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	345	4,223
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	43
Non-accrual loans	0	0
Loans not past due	3,296	3,057
Loans	3,296	3,100
Installment and Other Loans [Member ] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	0	43
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