Note 8 - Loans - Portfolio by Risk Rating (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Loans	$ 15,565,779	$ 15,075,481
Commercial Portfolio Segment [Member]
Loans	2,848,000	2,778,744
Real Estate Construction Portfolio Segment [Member]
Loans	675,112	579,864
Commercial Real Estate Portfolio Segment [Member]
Loans	7,459,316	7,275,262
Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans	4,581,695	4,436,561
Consumer Portfolio Segment [Member]
Loans	1,656	5,050
Pass [Member]
Loans	14,889,933	14,415,729
Pass [Member] | Commercial Portfolio Segment [Member]
Loans	2,553,148	2,528,944
Pass [Member] | Real Estate Construction Portfolio Segment [Member]
Loans	548,075	461,597
Pass [Member] | Commercial Real Estate Portfolio Segment [Member]
Loans	7,215,288	6,992,933
Pass [Member] | Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans	4,571,766	4,427,205
Pass [Member] | Consumer Portfolio Segment [Member]
Loans	1,656	5,050
Special Mention [Member]
Loans	394,033	477,071
Special Mention [Member] | Commercial Portfolio Segment [Member]
Loans	156,869	166,016
Special Mention [Member] | Real Estate Construction Portfolio Segment [Member]
Loans	122,299	113,687
Special Mention [Member] | Commercial Real Estate Portfolio Segment [Member]
Loans	114,653	196,454
Special Mention [Member] | Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans	212	914
Special Mention [Member] | Consumer Portfolio Segment [Member]
Loans	0	0
Substandard [Member]
Loans	281,813	182,681
Substandard [Member] | Commercial Portfolio Segment [Member]
Loans	137,983	83,784
Substandard [Member] | Real Estate Construction Portfolio Segment [Member]
Loans	4,738	4,580
Substandard [Member] | Commercial Real Estate Portfolio Segment [Member]
Loans	129,375	85,875
Substandard [Member] | Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans	9,717	8,442
Substandard [Member] | Consumer Portfolio Segment [Member]
Loans	0	0
Doubtful [Member]
Loans	0	0
Doubtful [Member] | Commercial Portfolio Segment [Member]
Loans	0	0
Doubtful [Member] | Real Estate Construction Portfolio Segment [Member]
Loans	0	0
Doubtful [Member] | Commercial Real Estate Portfolio Segment [Member]
Loans	0	0
Doubtful [Member] | Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans	0	0
Doubtful [Member] | Consumer Portfolio Segment [Member]
Loans	$ 0	$ 0