Note 15 - Regulatory Capital - Summary of the Bank's Compliance (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Savings Bank [Member]
Tier i (leverage) capital, capital level, amount	$ 667,882	$ 660,782
Tier i (leverage) capital, capital level, percent	9.69%	9.85%
Tier i (leverage) capital, requirement to be well capitalized, amount	$ 344,642	$ 335,512
Tier i (leverage) capital, requirement to be well capitalized, percent	5.00%	5.00%
Tier i (leverage) capital, excess, amount	$ 323,240	$ 325,270
Tier i (leverage) capital, excess, percent	4.69%	4.85%
Common equity tier i risk-based capital, capital level, amount	$ 667,882	$ 660,782
Common equity tier i risk-based capital, capital level, percent	12.66%	13.28%
Common equity tier i risk-based capital, requirement to be well capitalized, amount	$ 342,872	$ 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	$ 325,010	$ 337,396
Common equity tier i risk-based capital, excess, percent	6.16%	6.78%
Tier 1 risk-based capital, capital level, amount	$ 667,882	$ 660,782
Tier 1 risk-based capital, capital level, percent	12.66%	13.28%
Tier 1 risk-based capital, requirement to be well capitalized, amount	$ 421,997	$ 398,014
Tier 1 risk-based capital, requirement to be well capitalized, percent	8.00%	8.00%
Tier 1 risk-based capital, excess, amount	$ 245,885	$ 262,768
Tier 1 risk-based capital, excess, percent	4.66%	5.28%
Total risk-based capital, capital level, amount	$ 689,392	$ 681,727
Total risk-based capital, capital level, percent	13.07%	13.70%
Total risk-based capital, requirement to be well capitalized, amount	$ 527,496	$ 497,517
Total risk-based capital, requirement to be well capitalized, percent	10.00%	10.00%
Total risk-based capital, excess, amount	$ 161,896	$ 184,210
Total risk-based capital, excess, percent	3.07%	3.70%
Holding Company [Member]
Tier i (leverage) capital, capital level, amount	$ 600,730	$ 586,582
Tier i (leverage) capital, capital level, percent	8.72%	8.74%
Tier i (leverage) capital, requirement to be well capitalized, amount	$ 344,637	$ 335,616
Tier i (leverage) capital, requirement to be well capitalized, percent	5.00%	5.00%
Tier i (leverage) capital, excess, amount	$ 256,093	$ 250,966
Tier i (leverage) capital, excess, percent	3.72%	3.74%
Common equity tier i risk-based capital, capital level, amount	$ 558,848	$ 546,230
Common equity tier i risk-based capital, capital level, percent	10.60%	10.98%
Common equity tier i risk-based capital, requirement to be well capitalized, amount	$ 342,840	$ 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	$ 216,008	$ 222,848
Common equity tier i risk-based capital, excess, percent	4.10%	4.48%
Tier 1 risk-based capital, capital level, amount	$ 600,730	$ 586,582
Tier 1 risk-based capital, capital level, percent	11.39%	11.79%
Tier 1 risk-based capital, requirement to be well capitalized, amount	$ 421,957	$ 398,008
Tier 1 risk-based capital, requirement to be well capitalized, percent	8.00%	8.00%
Tier 1 risk-based capital, excess, amount	$ 178,773	$ 188,574
Tier 1 risk-based capital, excess, percent	3.39%	3.79%
Total risk-based capital, capital level, amount	$ 697,240	$ 682,527
Total risk-based capital, capital level, percent	13.22%	13.72%
Total risk-based capital, requirement to be well capitalized, amount	$ 527,446	$ 497,511
Total risk-based capital, requirement to be well capitalized, percent	10.00%	10.00%
Total risk-based capital, excess, amount	$ 169,794	$ 185,016
Total risk-based capital, excess, percent	3.22%	3.72%