Note 8 - Loans - Aging of Loan Portfolio (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Loans past due	$ 149,626	$ 131,462
Non-accrual loans	53,733	40,523
Loans not past due	15,384,586	14,944,019
Loans	15,534,212	15,075,481
Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	83,246	64,249
Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	8,116	20,281
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	4,531	6,409
Commercial Portfolio Segment [Member]
Loans past due	65,357	60,425
Non-accrual loans	30,443	19,381
Loans not past due	2,907,721	2,718,319
Loans	2,973,078	2,778,744
Commercial Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	26,282	24,681
Commercial Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	7,433	9,954
Commercial Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	1,199	6,409
Real Estate Construction Portfolio Segment [Member]
Loans past due	4,482	17,179
Non-accrual loans	4,482	4,580
Loans not past due	572,758	562,685
Loans	577,240	579,864
Real Estate Construction Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	0	5,846
Real Estate Construction Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	0	6,753
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	32,892	20,231
Non-accrual loans	11,859	9,928
Loans not past due	7,389,693	7,255,031
Loans	7,422,585	7,275,262
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	17,018	7,694
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	683	2,609
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	3,332	0
Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans past due	46,895	33,627
Non-accrual loans	6,949	6,634
Loans not past due	4,512,298	4,402,934
Loans	4,559,193	4,436,561
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	39,946	26,028
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	0	965
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	2,116	5,050
Loans	2,116	5,050
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