Note 15 - Regulatory Capital - Summary of the Bank's Compliance (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Savings Bank [Member]
Tier i (leverage) capital, capital level, amount	$ 673,084	$ 660,782
Tier i (leverage) capital, capital level, percent	9.66%	9.85%
Tier i (leverage) capital, requirement to be well capitalized, amount	$ 348,415	$ 335,512
Tier i (leverage) capital, requirement to be well capitalized, percent	5.00%	5.00%
Tier i (leverage) capital, excess, amount	$ 324,669	$ 325,270
Tier i (leverage) capital, excess, percent	4.66%	4.85%
Common equity tier i risk-based capital, capital level, amount	$ 673,084	$ 660,782
Common equity tier i risk-based capital, capital level, percent	12.79%	13.28%
Common equity tier i risk-based capital, requirement to be well capitalized, amount	$ 342,103	$ 323,386
Common equity tier i risk-based capital, requirement to be well capitalized, percent	6.50%	6.50%
Common equity tier i risk-based capital, excess, amount	$ 330,981	$ 337,396
Common equity tier i risk-based capital, excess, percent	6.29%	6.78%
Tier 1 risk-based capital, capital level, amount	$ 673,084	$ 660,782
Tier 1 risk-based capital, capital level, percent	12.79%	13.28%
Tier 1 risk-based capital, requirement to be well capitalized, amount	$ 421,049	$ 398,014
Tier 1 risk-based capital, requirement to be well capitalized, percent	8.00%	8.00%
Tier 1 risk-based capital, excess, amount	$ 252,035	$ 262,768
Tier 1 risk-based capital, excess, percent	4.79%	5.28%
Total risk-based capital, capital level, amount	$ 695,120	$ 681,727
Total risk-based capital, capital level, percent	13.21%	13.70%
Total risk-based capital, requirement to be well capitalized, amount	$ 526,312	$ 497,517
Total risk-based capital, requirement to be well capitalized, percent	10.00%	10.00%
Total risk-based capital, excess, amount	$ 168,808	$ 184,210
Total risk-based capital, excess, percent	3.21%	3.70%
Holding Company [Member]
Tier i (leverage) capital, capital level, amount	$ 606,844	$ 586,582
Tier i (leverage) capital, capital level, percent	8.71%	8.74%
Tier i (leverage) capital, requirement to be well capitalized, amount	$ 348,355	$ 335,616
Tier i (leverage) capital, requirement to be well capitalized, percent	5.00%	5.00%
Tier i (leverage) capital, excess, amount	$ 258,489	$ 250,966
Tier i (leverage) capital, excess, percent	3.71%	3.74%
Common equity tier i risk-based capital, capital level, amount	$ 564,466	$ 546,230
Common equity tier i risk-based capital, capital level, percent	10.73%	10.98%
Common equity tier i risk-based capital, requirement to be well capitalized, amount	$ 342,078	$ 323,382
Common equity tier i risk-based capital, requirement to be well capitalized, percent	6.50%	6.50%
Common equity tier i risk-based capital, excess, amount	$ 222,388	$ 222,848
Common equity tier i risk-based capital, excess, percent	4.23%	4.48%
Tier 1 risk-based capital, capital level, amount	$ 606,844	$ 586,582
Tier 1 risk-based capital, capital level, percent	11.53%	11.79%
Tier 1 risk-based capital, requirement to be well capitalized, amount	$ 421,019	$ 398,008
Tier 1 risk-based capital, requirement to be well capitalized, percent	8.00%	8.00%
Tier 1 risk-based capital, excess, amount	$ 185,825	$ 188,574
Tier 1 risk-based capital, excess, percent	3.53%	3.79%
Total risk-based capital, capital level, amount	$ 703,879	$ 682,527
Total risk-based capital, capital level, percent	13.37%	13.72%
Total risk-based capital, requirement to be well capitalized, amount	$ 526,274	$ 497,511
Total risk-based capital, requirement to be well capitalized, percent	10.00%	10.00%
Total risk-based capital, excess, amount	$ 177,605	$ 185,016
Total risk-based capital, excess, percent	3.37%	3.72%