Note 4 - Loans - Aging of Loan Portfolio (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Loans past due	$ 131,462	$ 106,057
Non-accrual loans	40,523	41,815
Loans not past due	14,944,019	13,889,731
Loans	15,075,481	13,995,788
Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	64,249	45,978
Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	20,281	14,491
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	6,409	3,773
Commercial Portfolio Segment [Member]
Loans past due	60,425	47,267
Non-accrual loans	19,381	18,805
Loans not past due	2,718,319	2,694,698
Loans	2,778,744	2,741,965
Commercial Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	24,681	25,494
Commercial Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	9,954	2,454
Commercial Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	6,409	514
Real Estate Construction Portfolio Segment [Member]
Loans past due	17,179	8,028
Non-accrual loans	4,580	4,872
Loans not past due	562,685	573,426
Loans	579,864	581,454
Real Estate Construction Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	5,846	0
Real Estate Construction Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	6,753	3,156
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	20,231	33,212
Non-accrual loans	9,928	10,611
Loans not past due	7,255,031	6,690,988
Loans	7,275,262	6,724,200
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	7,694	10,797
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	2,609	8,545
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	0	3,259
Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans past due	33,627	17,550
Non-accrual loans	6,634	7,527
Loans not past due	4,402,934	3,926,270
Loans	4,436,561	3,943,820
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	26,028	9,687
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	965	336
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	0
Non-accrual loans	0	0
Loans not past due	5,050	4,349
Loans	5,050	4,349
Installment and Other Loans [Member ] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	0	0
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