Note 8 - Loans - Aging of Loan Portfolio (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Loans past due	$ 93,423	$ 106,057
Non-accrual loans	47,200	41,815
Loans not past due	14,635,131	13,889,731
Loans	14,728,554	13,995,788
Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	33,117	45,978
Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	12,423	14,491
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	683	3,773
Commercial Portfolio Segment [Member]
Loans past due	61,611	47,267
Non-accrual loans	22,970	18,805
Loans not past due	2,606,450	2,694,698
Loans	2,668,061	2,741,965
Commercial Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	29,905	25,494
Commercial Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	8,736	2,454
Commercial Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	0	514
Real Estate Construction Portfolio Segment [Member]
Loans past due	4,629	8,028
Non-accrual loans	4,629	4,872
Loans not past due	589,187	573,426
Loans	593,816	581,454
Real Estate Construction Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	0	0
Real Estate Construction Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	0	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	19,626	33,212
Non-accrual loans	12,330	10,611
Loans not past due	7,115,973	6,690,988
Loans	7,135,599	6,724,200
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	2,926	10,797
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	3,687	8,545
Commercial Real Estate Portfolio Segment [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans past due	683	3,259
Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans past due	7,557	17,550
Non-accrual loans	7,271	7,527
Loans not past due	4,318,434	3,926,270
Loans	4,325,991	3,943,820
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Loans past due	286	9,687
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Loans past due	0	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	5,087	4,349
Loans	5,087	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