Note 4 - Loans - Portfolio by Risk Rating (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Loans	$ 15,644,396	$ 15,075,481
Commercial Portfolio Segment [Member]
Loans	2,836,833	2,778,744
Real Estate Construction Portfolio Segment [Member]
Loans	679,492	579,864
Commercial Real Estate Portfolio Segment [Member]
Loans	7,555,027	7,275,262
Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans	4,569,944	4,436,561
Consumer Portfolio Segment [Member]
Loans	3,100	5,050
Pass [Member]
Loans	14,927,044	14,415,729
Pass [Member] | Commercial Portfolio Segment [Member]
Loans	2,581,128	2,528,944
Pass [Member] | Real Estate Construction Portfolio Segment [Member]
Loans	593,196	461,597
Pass [Member] | Commercial Real Estate Portfolio Segment [Member]
Loans	7,202,568	6,992,933
Pass [Member] | Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans	4,547,052	4,427,205
Pass [Member] | Consumer Portfolio Segment [Member]
Loans	3,100	5,050
Special Mention [Member]
Loans	421,284	477,071
Special Mention [Member] | Commercial Portfolio Segment [Member]
Loans	141,344	166,016
Special Mention [Member] | Real Estate Construction Portfolio Segment [Member]
Loans	82,010	113,687
Special Mention [Member] | Commercial Real Estate Portfolio Segment [Member]
Loans	186,283	196,454
Special Mention [Member] | Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans	11,647	914
Special Mention [Member] | Consumer Portfolio Segment [Member]
Loans	0	0
Substandard [Member]
Loans	290,495	182,681
Substandard [Member] | Commercial Portfolio Segment [Member]
Loans	108,788	83,784
Substandard [Member] | Real Estate Construction Portfolio Segment [Member]
Loans	4,286	4,580
Substandard [Member] | Commercial Real Estate Portfolio Segment [Member]
Loans	166,176	85,875
Substandard [Member] | Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans	11,245	8,442
Substandard [Member] | Consumer Portfolio Segment [Member]
Loans	0	0
Doubtful [Member]
Loans	5,573	0
Doubtful [Member] | Commercial Portfolio Segment [Member]
Loans	5,573	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