Note 8 - Loans - Aging of Loan Portfolio (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Loans past due	$ 96,339	$ 98,709
Non-accrual loans	49,328	48,787
Loans not past due	12,918,200	12,771,581
Loans	13,014,539	12,870,290
Financing Receivables, 30 to 59 Days Past Due [Member]
Loans past due	46,770	38,396
Financing Receivables, 60 to 89 Days Past Due [Member]
Loans past due	241	11,526
Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Loans past due
Commercial Portfolio Segment [Member]
Loans past due	31,633	30,567
Non-accrual loans	15,916	14,296
Loans not past due	2,404,788	2,430,699
Loans	2,436,421	2,461,266
Commercial Portfolio Segment [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Loans past due		11,079
Commercial Portfolio Segment [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Loans past due	146	5,192
Commercial Portfolio Segment [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Loans past due
Real Estate Construction Portfolio Segment [Member]
Loans past due	9,033	11,213
Non-accrual loans	8,113	8,185
Loans not past due	578,894	667,592
Loans	587,927	678,805
Real Estate Construction Portfolio Segment [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Loans past due	920	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	43,795	42,995
Non-accrual loans	17,780	19,820
Loans not past due	6,566,459	6,439,700
Loans	6,610,254	6,482,695
Commercial Real Estate Portfolio Segment [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Loans past due	26,015	17,573
Commercial Real Estate Portfolio Segment [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Loans past due		5,602
Commercial Real Estate Portfolio Segment [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Loans past due
Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans past due	11,613	13,831
Non-accrual loans	7,519	6,486
Loans not past due	3,363,851	3,228,523
Loans	3,375,464	3,242,354
Residential Mortgage and Equity Lines Portfolio Segment [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Loans past due	4,094	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	265	103
Non-accrual loans
Loans not past due	4,208	5,067
Loans	4,473	5,170
Installment and Other Loans [Member ] | Financing Receivables, 30 to 59 Days Past Due [Member]
Loans past due	170	103
Installment and Other Loans [Member ] | Financing Receivables, 60 to 89 Days Past Due [Member]
Loans past due	95
Installment and Other Loans [Member ] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Loans past due