Note 8 - Loans - Aging of Loan Portfolio (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Loans	$ 15,690,689	$ 15,644,396
Financial Asset, 30 to 59 Days Past Due [Member]
Loans	18,826	118,402
Financial Asset, 60 to 89 Days Past Due [Member]
Loans	2,899	25,474
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans	1,513	4,982
Financial Asset, Past Due [Member]
Loans	91,020	216,542
Non-accrual loans	67,782	67,684
Financial Asset, Not Past Due [Member]
Loans	15,599,669	15,427,854
Commercial Portfolio Segment [Member]
Loans	2,867,437	2,836,833
Commercial Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans	17,010	52,601
Commercial Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans	599	3,182
Commercial Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans	1,249	2,947
Commercial Portfolio Segment [Member] | Financial Asset, Past Due [Member]
Loans	35,191	81,817
Non-accrual loans	16,333	23,087
Commercial Portfolio Segment [Member] | Financial Asset, Not Past Due [Member]
Loans	2,832,246	2,755,016
Real Estate Construction Portfolio Segment [Member]
Loans	664,495	679,492
Real Estate Construction Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans	0	6,257
Real Estate Construction Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans	0	0
Real Estate Construction Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans	0	0
Real Estate Construction Portfolio Segment [Member] | Financial Asset, Past Due [Member]
Loans	4,116	10,543
Non-accrual loans	4,116	4,286
Real Estate Construction Portfolio Segment [Member] | Financial Asset, Not Past Due [Member]
Loans	660,379	668,949
Commercial Real Estate Portfolio Segment [Member]
Loans	7,615,088	7,555,027
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans	228	45,186
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans	0	18,069
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans	0	2,035
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, Past Due [Member]
Loans	37,112	99,005
Non-accrual loans	36,884	33,715
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, Not Past Due [Member]
Loans	7,577,976	7,456,022
Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans	4,540,537	4,569,944
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans	588	14,315
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans	2,300	4,223
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans	264	0
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, Past Due [Member]
Loans	13,601	25,134
Non-accrual loans	10,449	6,596
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, Not Past Due [Member]
Loans	4,526,936	4,544,810
Installment and Other Loans [Member ]
Loans	3,132	3,100
Installment and Other Loans [Member ] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans	1,000	43
Installment and Other Loans [Member ] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans	0	0
Installment and Other Loans [Member ] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans	0	0
Installment and Other Loans [Member ] | Financial Asset, Past Due [Member]
Loans	1,000	43
Non-accrual loans	0	0
Installment and Other Loans [Member ] | Financial Asset, Not Past Due [Member]
Loans	$ 2,132	$ 3,057