Note 8 - Loans - Aging of Loan Portfolio (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Loans past due	$ 103,789	$ 98,709
Non-accrual loans	42,410	48,787
Loans not past due	13,543,857	12,771,581
Loans	13,647,646	12,870,290
Financing Receivables, 30 to 59 Days Past Due [Member]
Loans past due	28,212	38,396
Financing Receivables, 60 to 89 Days Past Due [Member]
Loans past due	26,486	11,526
Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Loans past due	6,681
Commercial Portfolio Segment [Member]
Loans past due	66,126	30,567
Non-accrual loans	17,118	14,296
Loans not past due	2,607,963	2,430,699
Loans	2,674,089	2,461,266
Commercial Portfolio Segment [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Loans past due	28,212	11,079
Commercial Portfolio Segment [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Loans past due	17,329	5,192
Commercial Portfolio Segment [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Loans past due	3,467
Real Estate Construction Portfolio Segment [Member]
Loans past due	4,922	11,213
Non-accrual loans	4,922	8,185
Loans not past due	592,096	667,592
Loans	597,018	678,805
Real Estate Construction Portfolio Segment [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Loans past due		3,028
Real Estate Construction Portfolio Segment [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Loans past due
Real Estate Construction Portfolio Segment [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Loans past due
Commercial Real Estate Portfolio Segment [Member]
Loans past due	25,543	42,995
Non-accrual loans	13,172	19,820
Loans not past due	6,554,711	6,439,700
Loans	6,580,254	6,482,695
Commercial Real Estate Portfolio Segment [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Loans past due		17,573
Commercial Real Estate Portfolio Segment [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Loans past due	9,157	5,602
Commercial Real Estate Portfolio Segment [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Loans past due	3,214
Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans past due	7,198	13,831
Non-accrual loans	7,198	6,486
Loans not past due	3,783,512	3,228,523
Loans	3,790,710	3,242,354
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Loans past due		6,613
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Loans past due		732
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Loans past due
Installment and Other Loans [Member ]
Loans past due		103
Non-accrual loans
Loans not past due	5,575	5,067
Loans	5,575	5,170
Installment and Other Loans [Member ] | Financing Receivables, 30 to 59 Days Past Due [Member]
Loans past due		103
Installment and Other Loans [Member ] | Financing Receivables, 60 to 89 Days Past Due [Member]
Loans past due
Installment and Other Loans [Member ] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Loans past due