Note 8 - Loans - Aging of Loan Portfolio (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Loans past due	$ 126,600	$ 131,462
Non-accrual loans	56,458	40,523
Loans not past due	15,481,683	14,944,019
Loans	15,608,283	15,075,481
Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	36,239	64,249
Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	12,529	20,281
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	21,374	6,409
Commercial Portfolio Segment [Member]
Loans past due	66,517	60,425
Non-accrual loans	27,125	19,381
Loans not past due	2,941,449	2,718,319
Loans	3,007,966	2,778,744
Commercial Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	18,968	24,681
Commercial Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	1,706	9,954
Commercial Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	18,718	6,409
Real Estate Construction Portfolio Segment [Member]
Loans past due	4,433	17,179
Non-accrual loans	4,433	4,580
Loans not past due	619,766	562,685
Loans	624,199	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	31,683	20,231
Non-accrual loans	10,896	9,928
Loans not past due	7,359,819	7,255,031
Loans	7,391,502	7,275,262
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	15,556	7,694
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	3,003	2,609
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	2,228	0
Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans past due	23,963	33,627
Non-accrual loans	14,004	6,634
Loans not past due	4,559,965	4,402,934
Loans	4,583,928	4,436,561
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	1,715	26,028
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	7,816	965
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	428	0
Installment and Other Loans [Member ]
Loans past due	4	0
Non-accrual loans	0	0
Loans not past due	684	5,050
Loans	688	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	4	0
Installment and Other Loans [Member ] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	$ 0	$ 0