Note 8 - Loans - Portfolio by Risk Rating (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Loans	$ 15,534,212	$ 15,075,481
Commercial Portfolio Segment [Member]
Loans	2,973,078	2,778,744
Real Estate Construction Portfolio Segment [Member]
Loans	577,240	579,864
Commercial Real Estate Portfolio Segment [Member]
Loans	7,422,585	7,275,262
Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans	4,559,193	4,436,561
Consumer Portfolio Segment [Member]
Loans	2,116	5,050
Pass [Member]
Loans	14,914,727	14,415,729
Pass [Member] | Commercial Portfolio Segment [Member]
Loans	2,726,364	2,528,944
Pass [Member] | Real Estate Construction Portfolio Segment [Member]
Loans	463,971	461,597
Pass [Member] | Commercial Real Estate Portfolio Segment [Member]
Loans	7,178,261	6,992,933
Pass [Member] | Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans	4,544,015	4,427,205
Pass [Member] | Consumer Portfolio Segment [Member]
Loans	2,116	5,050
Special Mention [Member]
Loans	437,396	477,071
Special Mention [Member] | Commercial Portfolio Segment [Member]
Loans	164,339	166,016
Special Mention [Member] | Real Estate Construction Portfolio Segment [Member]
Loans	108,787	113,687
Special Mention [Member] | Commercial Real Estate Portfolio Segment [Member]
Loans	158,354	196,454
Special Mention [Member] | Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans	5,916	914
Special Mention [Member] | Consumer Portfolio Segment [Member]
Loans	0	0
Substandard [Member]
Loans	182,089	182,681
Substandard [Member] | Commercial Portfolio Segment [Member]
Loans	82,375	83,784
Substandard [Member] | Real Estate Construction Portfolio Segment [Member]
Loans	4,482	4,580
Substandard [Member] | Commercial Real Estate Portfolio Segment [Member]
Loans	85,970	85,875
Substandard [Member] | Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans	9,262	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